Place an oven rack in the center of the oven. Preheat the oven to 325 degrees F. Spray a nonstick 9-by-5-inch loaf pan with vegetable oil cooking spray. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In a stand mixer fitted with a paddle attachment, beat the butter and sugar on high speed until light and fluffy, about 2 minutes. Beat in the eggs, one at a time, until smooth. Gradually add the flour mixture and mix until just incorporated. Reduce the mixer speed to low and add the mascarpone and vanilla.
Mix until incorporated.
Transfer the batter to the prepared loaf pan and bake for 70 to 75 minutes, or until a cake tester inserted into the center of the cake comes out with a few moist crumbs attached. Cool in the pan for 30 minutes.
Remove the cake from the pan and cool completely on a wire rack, about 2 hours.
For the icing: In a large bowl, using a high-powered hand mixer on medium speed, beat the egg whites until foamy.
Add the cream of tartar and beat until the mixture forms soft peaks. Gradually beat in the sugar. With the mixer running on low speed, drizzle the coffee extract into the egg whites. Increase the speed to high and beat for 3 to 4 minutes until smooth and thick.
Place the icing in a serving bowl.
Cut the pound cake into thin slices and serve with a dollop of icing.
