Preheat oven to 180 C / Gas 4. Grease two 23cm round tins with butter.
Beat eggs whites until stiff peaks form.
In a separate bowl, whisk egg yolks with 300g sugar until thick and pale yellow in colour. Sift flour and baking powder into the bowl. Add lemon juice and mix well.
Add the egg whites to the yolk mixture, folding in gently in a circular motion, taking care not to deflate the egg whites.
Tip 1/3 of the cake mixture into one of the prepared tins. Add cocoa powder to the remaining 2/3 of cake mixture, and mix till combined. Transfer the chocolate cake mixture to the other prepared tin. Bake in preheated oven 20 to 25 minutes. The lighter sponge may be ready sooner than the chocolate one. Test for doneness with a skewer.
For the filling, whisk together the creme fraiche and sugar until light and fluffy, about 5 minutes. Exact timing will depend on fat content of your creme fraiche. Stir in the 2 teaspoons lemon juice. The filling should become light and airy and should hold shape; it shouldn't be runny. Chill in fridge until ready to decorate.
When the cakes are done, run a knife along the edges of the tins. Invert on a wire rack to cool. Once cool, cut chocolate cake in half horizontally to make two rounds, with one being 1.5-2cm thick. Place the 1.5-2cm thick round on a serving plate.
Cut the remaining sponge (plain and chocolate) into 2cm cubes.
Cover chocolate base with filling.
Start forming your cake over the base: add a layer of cubed cake, and sprinkle with walnuts and pineapple, then cover with more filling.
Repeat layers, shaping the cake in the form of a volcano.
Finish with filling. Place in the freezer for 20 minutes (this step is optional, but will help with decorating later).
Decorate cake with melted chocolate pouring it over the top of cake with a teaspoon and letting it run down.
Chill in the fridge for 4 hours or overnight before serving.