In a saucepan, combine bittersweet and semisweet chocolate, espresso, 1/4 cup sugar and butter; heat over medium heat, stirring, until chocolate is melted and mixture is smooth. Remove from heat and stir for 5 minutes to cool.

In a bowl, using an electric mixer, beat egg whites until almost stiff. Add 1 Tbsp sugar and beat until stiff peaks form. Set aside.

In a large bowl, lightly whisk egg yolks. Whisk in a little of the chocolate mixture. Gradually whisk in the remaining chocolate mixture. Fold in beaten egg whites until well mixed but still fluffy.

Wrap the base of a buttered 7-inch springform pan with foil. Pour batter into the pan and cover with a piece of buttered foil, making sure pan is well sealed.

Set rack in the bottom of the pressure cooker. Pour in 2 cups water for steaming. Fold a 2-foot long pieces of foil folded several times to make a strip strong enough to lift the pan. Center pan on midpoint of strip and fold the ends together to make a handle. Lower the pan into the cooker.

Lock the lid in place and bring the cooker up to full pressure over high heat. Reduce heat to medium-low, just to maintain even pressure, and cook for 20 minutes. Remove from heat and allow pressure to drop naturally. Using foil handle, lift pan out of the cooker onto a cooling rack. Remove foil lid and let cake cool completely.

Release cake from pan and dust with confectioner's sugar. Cut into wedges and serve with whipped cream and an espresso bean on top of each wedge.