Place an oven rack in the center of the oven and preheat the oven to 350 degrees F. Coat an 8-inch by 8-inch pan with cooking spray. Line with parchment paper and spray parchment with cooking spray.
Melt the butter in a medium saucepan over medium-low heat. Add the sugar and stir until the mixture is hot, 2 to 3 minutes. Transfer mixture to a mixing bowl. Stir in cocoa, espresso powder, vanilla, salt, and baking powder until well mixed. Stir in eggs until blended. Stir in flour and chocolate chips. Pour the batter into the prepared pan and smooth the top. Bake until the edges are set and a toothpick inserted in the center comes out with moist crumbs, 20 to 24 minutes.
Cool in the pan on a wire rack for 15 minutes. Run a knife around the edges and turn out onto a wire rack. Cool completely.
To make the frosting: Beat the butter and mascarpone together in a large mixing bowl until smooth and well combined. Add in the melted chocolate and vanilla and beat well. Slowly add in the confectioner's sugar and salt and beat until smooth. If the mixture looks curdled, keep beating until it becomes thick and smooth. Spread the frosting over the brownies, cut into 9 squares and serve.
Ingredient note: If you cannot find dark cocoa powder, you can use half Dutch-processed and half unsweetened natural cocoa.
Cook's Note: These brownies are a dark chocolate ooey, gooey delight enhanced by a kiss of espresso powder and finished with creamy mascarpone frosting. The key steps to making this dish unique and successful is the use of the highest quality ingredients, especially the chocolate; heating the butter and sugar together in the first part of the recipe; a light hand when stirring in the flour; a pinch of sea salt in the frosting, and finally not over baking the brownies!