Preheat the oven to 350 ℉. In a large mixing bowl, add the powdered sugar and pour the melted butter overtop. Stir to dissolve the sugar for 1 minute.
Meanwhile, in a separate bowl, melt the chopped dark chocolate in the microwave in 30-second intervals, stirring in between, until fully melted. Pour into the mixing bowl along with the vanilla extract, whole eggs, and egg yolk, and stir until combined.
Sift in the flour, espresso powder, baking powder, and salt, and gently fold in until just combined--do not overmix! Gently fold in the chocolate chips.
Grease and line an 8x8 brownie pan with nonstick cooking spray, then lightly spray the parchment paper. Scrape the brownie batter into the pan and spread into a smooth, even layer. Bake for 30-35 minutes; the center will still jiggle, but a toothpick will only have a few wet crumbs. Allow the brownies to fully cool in the pan on a wire cooling rack, then lift out of the pan by the parchment paper and cut into desired serving sizes.
