Preheat oven to 350°F and spray 9x13 baking dish with nonstick cooking spray (or line with parchment paper).
In a microwave safe dish combine the butter and chopped baking chocolate*. Microwave for 30 seconds, stir, and repeat until the butter and chocolate is melted and smooth. Set aside. (¾ cups unsalted butter, 4 oz unsweetened baking chocolate)
While butter mixture is cooling, in a small bowl combine the flour, baking powder, and salt. Set aside. (1 cup flour, ½ teaspoon baking powder, ¼ teaspoon salt)
In a stand mixer mix together the butter mixture (it’s okay if it’s still warm) and the sugar. Stir in the espresso powder now if using. (2 cups sugar, 1 teaspoon espresso powder)
Add the eggs one at a time, letting each egg become fully incorporated into the batter before adding the next. Once the eggs are all added, mix in the vanilla. (4 large eggs, 1 teaspoon vanilla)
By hand, with a large rubber spatula or mixing spoon, fold in the flour mixture. Folding in the flour will help the brownie batter keep the air in it.
Fold in the chocolate chips. (1 cup chocolate chips)
Pour into the prepared baking dish and bake for 25-30 minutes or until a toothpick comes out clean.
Let cool completely before cutting (if you can!).