Custard: In a saucepan over low heat, or in a metal bowl set over a saucepan of simmering water, whisk together the egg yolks and sugar. Stir in the cream. Cook and stir until the mixture is very thick. A few drops dribbled onto the surface should be visible. Remove from heat and stir in the butter until completely incorporated. Place a sheet of plastic wrap directly on the surface and refrigerate overnight.
Cake: Preheat the oven to 200 degrees C. Grease a 23cm springform cake tin.
In a glass or metal bowl, whip egg whites with an electric mixer to firm peaks. In a separate bowl, stir together the almonds, baking powder and icing sugar. Fold into the egg whites until well blended. Pour the batter into the prepared cake tin.
Bake for 30 minutes in the preheated oven until the top of the cake springs back when pressed lightly.
To Serve: Cool cake then remove from the cake tin. Spread the custard on top and serve.