Preheat oven to 180 degrees C. Line a baking tray with baking paper and butter it.
Whisk the egg whites in a bowl until stiff peaks form.
In a separate bowl, beat egg yolks while gradually adding caster sugar until thick and pale. Carefully fold in 1/4 of the egg whites. Sift in the flour and baking powder and mix well.
Gently fold in the remaining egg whites.
Drop heaped teaspoonfuls of the mixture on the baking paper to form biscuits. Make a few biscuits in a ladyfingers shape for the legs of the tortoise. Bake 7-8 minutes, or until golden at the edges. Let cool.
Beat the creme fraiche or sour cream with the caster sugar until light and fluffy, about 5 minutes. Beat in the cocoa powder then refrigerate until ready to make cake.
On a tray, make a circle shape out of biscuits, securing them with cream filling. Add another layer of biscuits and more filling. Continue until you have made a mound in the shape of a tortoise shell, and cover with remaining filling.
Use the ladyfinger shaped biscuits to make four legs. Sandwich three biscuits together to make the head.
Sprinkle the cake with diced walnuts to decorate. Use walnut pieces to make the eyes and nostrils.