Preheat the oven to 350º F . Line a 13x9 inch metal pan with nonstick foil or parchment.
Melt the butter, 8 ounces (1 ⅓ cup) semi-sweet chocolate chips and the unsweetened chocolate in a saucepan over low heat or in the microwave. Let cool slightly.
Whisk the eggs, vanilla, and sugar together in a large mixing bowl. Stir the warm chocolate mixture into the egg mixture and let cool to room temperature.
Stir the flour, baking powder, salt and espresso powder together in another bowl then add to the cooled chocolate mixture. Stir in the Oreos and remaining chocolate chips.
Bake for 20 minutes, then rap the baking sheet against the oven shelf to force the air to escape from between the pan and the brownie dough. Bake for about 10 minutes more, or until a toothpick comes out clean. Do not overbake!
Let cool, then chill brownies. Lift from pan and cut into squares.
Notes
For a half batch, use an 8-inch square pan and bake at 325F instead of 350. The brownies should be done in 28 to 30 minutes.