In a small bowl combine the espresso and bourbon and set aside.
Add egg whites to an electric mixer and beat on high until stiff peaks form. Remove to a large bowl and set aside.
Meanwhile, over a double boiler, whisk together the egg yolks, sugar, and salt. Over low heat, whisk constantly until the temperature reaches 160ºF, about 5-7 minutes.
Add the egg yolk mixture to the mixer and on medium speed, beat until light and creamy, about 4-5 minutes. Add the mascarpone cheese and vanilla extract and beat until thick and smooth. The cheese will be lumpy at first but keep beating until smooth. Fold in the egg whites.
Slice the Plain Jane Cake into small slices and very briefly dip one side of the cake into the coffee mixture and place in a prepared square baking pan.
Repeat until the first layer covers the entire bottom of the pan. Top with half of the custard mixture and dust with cocoa powder. Repeat with a second layer, ending with a dusting of cocoa powder.
Cover the pan with plastic wrap and let chill in the fridge for 4 hours or overnight. Cut into slices and serve.
You can make this with strong coffee instead of espresso.