Preheat oven to 180 C / Gas 4. Line a baking tray with parchment, then butter it well.
Whisk egg whites until stiff peaks form.
In a separate bowl, beat egg yolks, gradually adding 200g sugar, till thick and pale yellow in colour. Add 1/4 of the egg whites to the yolk mixture, folding in gently. Sift flour with baking powder and add to the mixture; mix well.
Add the rest of the egg whites to the mixture, folding in gently with spatula in a circular motion, taking care not to deflate the egg whites.
Drop mixture on the baking tray using teaspoon, leaving a 5cm (2 in) space in between each round biscuit. (Make two biscuits shaped like ladyfingers, to later make 4 feet by breaking them in half.) Bake 7 to 8 minutes, or until edges look golden. Let cool.
For the filling, beat chilled creme fraiche with sugar until it becomes light and fluffy, about 5 minutes. Add cocoa and blend it in. Put bowl with filling in the fridge until ready to form cake.
Start forming your cake by making round or oval layers out of the cooled biscuits. Spread a generous amount of filling on top of each layer. Continue, making a mound that looks like a tortoise shell. Set aside 2 to 3 biscuits for the head.
Break the two elongated biscuits in half. Place at the front and back of the mound to make feet. Make head by stacking 2 to 3 biscuits on top of each other. Decorate head with filling. Spread the rest of filling all over the cake, then sprinkle cake with chopped walnuts. Make eyes and nostrils out of walnut pieces.
Chill in the fridge for 6 hours or overnight before serving.