Whisked egg yolks, sugar and usually alcoholic liqueur, whipped to a ‘ribbon stage’ over hot water, is called a ‘sabayonne’. Sabayonne works perfectly well as a dessert on its own, served with strawberries or other fruit.
Preheat the oven to 180°C/fan 160°C/gas 4. Grease a 17cm x 17cm square baking tin with butter, then line it with 2 rectangular pieces of greaseproof paper. This is so you can lift the cake out of the tin when it has cooked, so overlap the pieces and leave 5cm at either side which will be your ‘handles’.
Place the egg whites in a large clean bowl (any grease at all will stop the whites whisking properly) and add half the caster sugar. Using an electric beater, whisk until they form stiff peaks and then set aside.
Bring a medium-sized saucepan of water to simmering point, and place a large bowl over the pan so the base of the bowl does not touch the water below (this is a bain-marie). Put the egg yolks and the remaining 20g of caster sugar into the bowl and whisk until you reach the ribbon stage (forming thick ‘ribbons’ that fall off a spoon). Remove the bowl and put to one side.
Place a second bowl on the saucepan. Put the chocolate and butter in the bowl and let them melt, stirring occasionally.
Take the egg yolk mixture and stir in the melted chocolate and melted butter, then fold in the whisked egg whites.
Pour the cake mixture into the lined baking tin and place in the preheated oven for 12 minutes.
Meanwhile, using a hand whisk, whip the double cream and icing sugar together until they form soft peaks, then pour in the coffee and stir well to mix.
Allow the cake to rest for 10 minutes in the tin − this will allow it to set slightly. After this time, carefully lift the edges of the greaseproof paper to remove the cake from the tin. Slice it into 4 squares and serve at room temperature with a generous dollop of the coffee cream.