1. Preheat your oven to 180°c/160°c fan. Grease a loose-bottom cake tin with a little butter. I used a 23cm cake tin, but if you want to double the recipe you’ll need a 28-30cm tin.
2. Break up the chocolate into small pieces and add to a heatproof bowl along with the butter. Melt over a pot of gently simmering water (making sure the bottom of the bowl doesn’t touch the water). Once melted, set aside to cool.
3. Add the egg whites and a pinch of salt (to help stabilise the eggs) to a different bowl and whisk until they form soft peaks. Set aside in the fridge.
4. Add the sugar and egg yolks to another bowl and whisk until light and fluffy.
NOTE 1: all the whisking is best done with an electric whisk or stand mixer (you can do by hand but it will take longer).
NOTE 2: whisk the egg whites first so you don’t have to wash the whisk before doing the yolks. A little white in the yolk is fine. Conversely, a tiny amount of yolk in the whites would prevent them from aerating properly.
5. Once the chocolate and butter mixture has slightly cooled, add it to the yolk and sugar mixture. Mix thoroughly, using a spatula, until completely combined.
Add in the ground almonds and mix to combine again. At this stage the mixture will thicken a lot but don’t worry. Everything comes good in the next step.
6. Gently fold in the egg whites. Start by taking about ⅓ of the egg whites and mixing it through, without too much care, until fully combined. Take another ⅓ and, this time, gently fold through until combined. Finally, gently fold through the remaining egg whites until you’re left with one uniform mixture.
7. Pour everything into the greased cake tin. Bang it gently on the work surface to level it out. Transfer to the oven.
8. Cook for 25-30 mins. It should form a slight crust when it is ready. I found 25 minutes was perfect but it'll vary slightly depending on your oven and the size of the cake tin.
NOTE: using a skewer to test this cake won’t really work as it will always remain very moist.
9. Allow to cool for about 30 mins before removing from the cake tin. Dust with a little icing sugar to finish. For maximum pleasure, serve while still warm with a scoop of vanilla ice cream.