Buying a 100th birthday present is a difficult task. Any birthday present purchase can present problems, at least for me. I want to make sure I get it right. Something that will be valued, something that will be loved. Something that will last.
A present for someone completing a century on the planet is a little different. Concepts like ‘lasting a lifetime’ take on a completely different hue. It’s almost impossible to get something useful that they don’t already have. Beautiful things seem trivial, ephemeral.

When my gran, my closest and only surviving grandparent, turned 100 a few weeks ago, I wanted a gift to express love, but just buying an expensive thing doesn’t cut it with grandparents.

Born in 1912, the year of Titanic, she went to Jersey to celebrate her 21st birthday in 1933, hanging out with chaps in blazers and lasses in swimming costumes. Her older brothers fought in the First World War, and her husband in the Second. She was a smart and funny white-haired lady when I was born, her first grandchild, and she still is. She’s an incredible woman, as sharp and funny as ever, and coming up with a suitable gift was daunting.

We gathered as a family for a weekend of celebrating – dinners, lunches and finally an afternoon tea with cards and presents. I bought her a painted silk scarf so there was something to unwrap. But I think my main contribution to the weekend was helping with the birthday cake.

This was a real joint effort between my mum, my sister and me. Although it needed to be celebratory, and bear decoration, she didn’t want either chocolate cake or fruit cake. As she doesn’t have much of a sweet tooth, we went for lemon sponge, but that posed the question of how best to decorate it.

Having done a bit of research on smitten kitchen and sweetapolita, I decided what we really needed was Swiss buttercream. The reason for using this is that it is very stable, and can be made ahead; it is not too sweet and sickly, unlike quick buttercream or fondant; and it can be made into a smooth surface to support piped icing decorations.

Swiss buttercream is made of a mixture of meringue and softened butter. The meringue is made using the Swiss method, dissolving the sugar into warmed egg whites before beating (as opposed to the French method:whisking sugar directly into whipped egg whites; or the Italian: pouring a hot sugar syrup into whipped egg whites).

You begin by measuring the sugar and egg whites into a large heatproof bowl. I used egg whites from a carton – I know there are virtually no problems now from British eggs, and you are heating them in this recipe, but I didnt want to take any chances with a 100 year old consumer. In any case, the carton whites (available as Two Chicks in a number of supermarkets in the UK) are very convenient for weighing out a precise amount.

This mixture is then heated over a barely simmering pan of water, stirring constantly, until it reaches 60c and the sugar has all dissolved into the whites. You can check this by rubbing the mixture between your fingertips to check that all the gritty sugar crystals are gone.

Then the warmed egg white syrup is whisked into stiff meringue peaks. We made two batches of the buttercream, fearing that my mum’s handheld electric mixer might not handle a full batch. Neither batch reached stiff peaks at this stage – both were still flowing a little, one more than the other, although the mixture was quite thick and opaque.

We pressed on anyway, despite the lack of stiff peaks, and it turned out fine. I don’t think there was any difference in texture between the two final batches. The process of adding the butter tends to deflate the mixture at first anyway. As long as the mixture is very thick and holds a ribbon, it should still be fine, even without peaks.

Once the meringue is thick, and has cooled to not far above room temperature, you can start to whisk in the butter. The butter should be cubed and a room temperature, so you can easily create an impression in it with your finger, but not so soft it starts to get shiny and greasy.

Whisk in one cube at a time. The mixture will deflate, and become thinner, perhaps quite soupy. It might also split and curdle. Keep whisking and adding butter. Once the butter is almost all incorporated, the texture suddenly changes, and it snaps to a thick mixture, that holds its shape very cleanly.

This can be kept in the fridge for several days, or frozen. When chilled, the texture is much like that of butter – quite firm and almost waxy. For this reason, you should make sure cakes frosted with Swiss buttercream are served at room temperature. To bring the the buttercream back to a working consistency, bring it slowly to room temperature, or give a couple of very low power bursts in the microwave, just as you would do when softening butter from the fridge. To help the process along, return the batch to a bowl and use an electric whisk or stand mixer to whip the buttercream until it is soft and spreadable again.

For the final cake, we layered my mum’s 8 inch square sponge layers, flavoured with lemon zest, with most of a jar of bought lemon curd.

I trimmed the edges square, and covered the cake with a crumb coat of buttercream. After chilling for about an hour, the cake surface was firm, like a block of cold butter, and we could layer on the rest of the buttercream.

Starting at the sides, this task looks a lot like plastering – apply a decent sized blob, and smooth it to an even layer. The great thing about Swiss buttercream is that it can be spread and smoothed out many times, and it will hold its texture. After covering it with a more or less even layer of frosting all over, I used a long palette knife to drag along the sides and over the top to make the surface as smooth and flat as possible. Then the cake went into the fridge overnight.

The next morning, we mixed up some fairly thick royal icing with yellow colour paste, and my sister, practiced from many school holidays working in a Thorntons shop, piped 100 on the top of the cake, and scrolls in the corners.

The final touch was to add my mum’s crystallised primroses, made with flowers from her garden, brushed with egg white and dusted with sugar.

The final cake went down well – it was moist in the centre, not too sweet. And a fitting cake to bear candles for a 100 year old to blow out.