Sift flour with baking soda, baking powder, and salt. Set aside

Combine coffee and buttermilk.

Cream butter until light and smooth. Add sugar and cream until fluffy. Add eggs one at a time, beating until the mixture is pale colored and smooth.

Beat in the melted chocolate until the batter is an even color.

Add half of the dry ingredients to your egg mixture, followed by the wet ingredients, and the remaining dry ingredients. Beat each addition until just smooth. (Your batter should resemble a very light and fluffy chocolate mousse.)

Spread the batter into a greased 8x8 pan and bake 20-27 minutes, until cake is nicely domed on top, is firm and springy to the touch, and only a few crumbs remain on a toothpick inserted at the center.

Set the cake, in the pan, on a wire rack and cool completely, 1-2 hours.

As the cake is cooling, make your custard. In a double boiler set to a simmer, whisk egg yolks with sugar. Heat, whisking constantly, until the sugar dissolves and the mixture thickens, 10 minutes.

Remove the egg yolks from the heat and beat them with an electric hand mixer until the custard is cool, thick, and pale yellow. Add the Mascarpone, a quarter cup at a time, beating after each addition until smooth. Add the rum and beat until incorporated. Set the custard aside.

In a chilled bowl, beat your chilled heavy cream until stiff peaks form. Gently fold the whipped cream into the Mascarpone custard.

Chill the custard mixture in the refrigerator until the cake is completely cool.

Once the cake is completely cooled, spread the Mascarpone rum custard over top of the cake in the pan. Place the cake in the refrigerator for at least 1 hour to let the custard set. (This can be done up to a day ahead of time.)

When you are ready to serve your cake, dust the top with a generous layer of cocoa powder.

Cut and serve.