1 Butter and flour a square cake pan. Finely chop the walnuts. Separate the whites from the yolks. Pour the rum over the breadcrumbs.
2 Preheat the oven to 180°c (th6). Mix the yeast and nuts in the breadcrumbs. Whisk the egg whites until stiff, with a pinch of salt.
3 Whisk the egg yolks with 150g of sugar until the mixture whitens. Stir the nut mixture into the beaten eggs. Then add the egg whites, lifting the mass well. Pour into the mould and put in the oven for 30 minutes.
4 Let the cake cool completely and cut it in half in the thickness. Whisk the mascarpone with the remaining sugar and the coffee. Spread this cream on one side of the cake and close it. Cover and bake for 4 hours. Decorate before serving.