Preheat oven to 350°. Set aside a square cake pan with 2" high sides for baking the custards.
In a small bowl, combine the Kahlua and espresso powder, stirring until powder is dissolved. Set aside.
In a small saucepan, heat the whipping cream, milk and sugar over medium high heat until it just reaches a boil. Remove from heat and add the chopped chocolate and stir until completely dissolved. Stir in the Kahlua mixture. Transfer 1/2 cup of hot custard to a glass measuring cup with a spout.
Whisk the egg yolks together and then slowly drizzle in the reserved custard in the measuring cup, whisking constantly. (Adding the hot milk to the eggs very slowly will temper the eggs and prevent them from scrambling). Add the egg mixture back to the larger pot with the chocolate and cream. Stir to combine.
Use 1/2 cup ramekins or other oven-safe glass containers for the pots de creme. Pour the custard mix into the glass measuring cup (you may have to do this in several batches, depending on how large your measuring cup is). Set a fine mesh strainer over the first ramekin and pour the custard mixture through. Fill nearly to the top. Continue filling the remainder of the ramekins in this fashion.
Carefully place ramekins in the square cake pan. Place the pan in the oven. Fill the cake pan with hot water that comes about halfway up the sides of the ramekins. Bake for 30-40 minutes or until custards are set, but still slightly jiggly in the center. Remove from the oven and cool to room temperature. Cover and refrigerate for several hours or overnight.
Make Kahlua Whipped Cream
Using a hand mixer whip the cream until it just starts to get some volume but not to the soft peak stage. Add the powdered sugar and continue to whip to soft peaks. Add the cornstarch and kahlua and whip to stiff peaks.
Dollop whipped cream on each pots de creme OR fit a piping bag with a large tip and fill the bag with whipped cream. Pipe the cream onto the individual custards. Garnish as desired.