Position rack in middle of oven. Preheat oven to325°F (160°C).
Using separate bowls, separate egg whites from the yolk. Stir once, then set aside egg yolks (Store egg whites to make an omelet or souffle).
Place on low medium heat, a medium pot, add heavy whipping cream and allow to simmer. Once it starts simmering slightly (small bubbles will appear around the inner rim in approx. 3 minutes). Add bittersweet chocolate, semisweet chocolate, and sugar, stir until chocolate melts. Do not let the mixture boil. Turn off and remove melted chocolate mixture from the heat.
Add the coffee concentrate and pure vanilla extract to chocolate mixture.
Next add slowly by whisking into the egg yolk about 2 tablespoons of chocolate mixture to temper the eggs. Slowly whisk in the remaining chocolate mixture. (Make sure to constantly mix as you pour and do so slowly. Pouring hot liquid fast with egg yolks can cause them to potentially become scrambled). You will see that the mixture will thicken slightly as you stir.
Pour chocolate custard into the ramekins. Before placing ramekins in a deep dish pan or hotel pan lay a sheet of paper towel on the bottom (keeps ramekins from sliding around). Then add hot water reaching halfway up ramekins sides.
Place pan in the oven and bake custards for approximately 35 minutes. They should be set at the center, firm and smooth when done.
Take pan out of oven, then remove ramekins from water bath. Allow to cool for about 10 minutes then place chocolate custards in refrigerator until completely chilled (approximately 1 hour 20 minutes). Serve it in its simple form or garnish with whipped cream and shaved chocolate shavings.