How to Make It
Preheat the oven to 300°. Butter and flour an 8-inch round cake pan.
In a large heatproof bowl set over a pan of simmering water, combine the chocolate with the butter, espresso and rum and stir until melted and smooth. Remove the bowl from the heat and stir in the sugar and ground almonds. Let the chocolate mixture cool slightly, then beat in the egg yolks, 1 at a time.
In a clean stainless-steel bowl, beat the egg whites to stiff peaks. Stir 1/3 of the beaten whites into the chocolate mixture, then fold in the remaining whites until fully incorporated. Pour the batter into the prepared pan and bake for 45 minutes, or until a toothpick inserted in the center of the cake comes out clean. Let the cake cool completely on a rack before inverting it onto a platter.