In a heat-proof bowl, whisk together egg yolks and 3/4 cup sugar until well blended.

Place bowl over a double boiler on low heat, whisking constantly until mixture begins to boil. Continue to whisk until yolk is a pale yellow color and thick enough to hold its shape for a few seconds in the bowl if dropped from the whisk. Remove from heat, and allow to cool until just warm to the touch.

Whisk in mascarpone until smooth. Cover tightly and refrigerate for one hour.

Whisk cold heavy whipping cream until it thickens and turns into whipped cream. Mix in vanilla.

Gently fold in whipped cream to the chilled custard until completely incorporated, then return custard to the refrigerator until ready to use.

Preheat oven to 400 F.

Place egg whites in the bowl of an electric stand mixer and beat on high speed until soft peaks start to form. Slowly add 2 tablespoons of the sugar and continue beating until stiff and glossy.

In another bowl, beat egg yolks and remaining sugar until thick and very pale in color.

In a small bowl, whisk together flour and baking powder.

Gently fold half of the egg yolk mixture into the egg white mixture. Gently fold in the flour mixture followed by the remaining egg white mixture.

Fit a large pastry bag with a plain 1/2 inch round tube and transfer mixture to the bag. Pipe the batter evenly between 18 cupcake liners.

Bake for 13 minutes or until a toothpick comes out clean. Cool to room temperature.

In a small bowl, combine coffee and rum. Poke holes into each cupcake with a toothpick or skewer.

Drizzle 1 tablespoon of the coffee/rum mixture over each cupcake. Wait for it to be completely absorbed, then add another 1 tablespoon to each cupcake.

Remove the custard from the refrigerator and spread it evenly over the soaked cupcakes. Cover and refrigerate cupcakes 4 to 6 hours or until set.

Whisk cold heavy whipping cream until it thickens and turns into whipped cream. Pipe or spread the whipped cream over the mascarpone mixture on each cupcake. Sprinkle with cocoa powder and top with chocolate shavings.