Preheat the oven to 350 degrees.
Generously grease an 8-inch cake pan and line with parchment paper. Also grease the parchment paper.
Grind the walnuts and breadcrumbs together in a food processor. The walnuts should be quite fine, but don’t process too long or they’ll lose texture and become oily.
Separate the egg whites and yolks. In the top of a double boiler, over simmering water, whisk the yolks and sugar together until they have tripled in volume and become thick and very pale yellow.
Take the yolks off the heat and stir in the nut mixture and chocolate.
Beat in the butter a tablespoon at a time until completely incorporated.
Stir in the coffee or espresso.
Whisk the egg white to stiff peaks.
Stir a small amount of the beaten whites into the batter to loosen it. Add the batter to the remaining egg whites and gently fold it in.
Pour the batter into the prepared cake pan.
Bake until a tester inserted in the center comes out clean, approximately 35 minutes. Note – the cake will pull away from the sides of the pan during baking.
Remove the cake from the oven and let cool completely on a wire rack before transferring to the serving plate.
Decorate with sifted confectioner’s sugar and additional walnuts, if desired. Serve with whipped, sweetened cream.