Preheat oven to 180 degrees C and butter two 23cm round cake tins.
Beat the egg whites until stiff peaks form. In a separate bowl, whisk yolks with 300g sugar until thick and pale. Sift in the flour and baking powder, add the lemon juice and mix well. Gently fold in the egg whites.
Pour 1/3 of the batter into one of the cake tins. Add the cocoa powder to the remaining batter, mix well and pour into the other tin. Bake 20-25 minutes (the plain cake will be done first) or until they test done.
Make the filling: whisk together the sour cream or creme fraiche and 200g sugar until light and fluffy. Stir in the lemon juice. Filling should not be runny. Chill until ready to use.
When cakes are done, invert on a wire rack and let cool. Cut the chocolate cake in half horizontally to make two layers. Place one of the layers on a serving plate. Cut the remaining layer and the plain cake into 2cm cubes.
Cover the chocolate base with a layer of filling. Add a layer of cubed cake, sprinkle with diced pineapple and walnuts, then cover with more filling. Repeat layers, forming the cake into the shape of a volcano. Finish with a coating of filling. Freeze 20 minutes.
Decorate the cake by drizzling melted chocolate over the top and letting it run down like lava. Chill four hours before serving.