Butter a 10 by 2-inch cake pan. Preheat oven to 350 degrees F.
In a food processor, pulse the nuts until coarsely chopped. Reserve. Process the chocolate until coarsely chopped. Add to the chopped nuts.
In a mixing bowl, combine the egg yolks and 1/4 cup sugar. Beat until light and ribbon-y.
In another mixing bowl, beat the egg whites to soft peaks. Slowly add the remaining 1/4 cup of sugar and continue to beat until stiff peaks form.
Fold the nuts and chocolate into the egg yolk mixture. Carefully fold the egg white into the yolk mixture. Fill the cake pan 2/3 full. Bake for 20 minutes or until center is springy. Remove from the oven and cool on a baking rack. Dust with cocoa powder or decorate with chocolate drawings.