Preheat the oven to 350 degrees. Line an 8 or 9 inch square baking dish with a parchment sling and spray with a neutral cooking spray. Set aside.

In a medium sized pot, melt the butter on the stove top over medium heat. Remove from heat, then whisk in brown and white sugar until smooth and creamy, about 1-2 minutes.

Add the eggs and egg yolk, one at a time, whisking well after each addition. Stir in the vanilla.

Add the cocoa powder, flour, salt, and baking powder and stir until just combined.

Pour the mixture into the prepared pan and bake for 30-35 minutes or until center is just set. A toothpick inserted in the center should come back with a few moist crumbs attached.

In a measuring cup, mix together the espresso and dark rum.

Prick the warm brownies all over with a fork. Slowly pour the rum mixture over the brownies, allowing it to soak in. You don't want it to be puddled on top. You might have leftover coffee mixture, which is fine. Allow the brownies to cool.

Combine the egg yolks, rum, and sugar in a double boiler or a bowl set over a pot of gently simmering water. Use a hand mixer to beat the eggs until fluffy and light in color, about five minutes. You have to beat continually to avoid scrambling the eggs. Gently fold in the mascarpone cheese and allow the mixture to chill in the fridge for about 10 minutes.

In another bowl or electric mixer, beat the whipped cream until stiff peaks have formed. Gently fold it into the mascarpone mixture. The whipped cream will deflate slightly.

Spread the whipped cream mixture evenly over the cooled brownies. Dust with sifted cocoa powder and grated chocolate.

Chill, then serve. Cover with saran wrap and chill for at least one hour or overnight. Cut into bars and serve.