Preheat the oven to 350 degrees. Spray a 9-inch square baking pan with cooking spray and line the bottom of the pan with parchment paper. Spray the paper with cooking spray and set pan aside.

Combine the chocolate chips and butter in a microwave-safe bowl and heat for 1 1/2 minutes, stopping every 30 seconds to stir, until chocolate and butter are melted and smooth. Set aside to cool for 10 minutes.

In a medium mixing bowl, whisk together the eggs, sugars, vanilla, cocoa powder, and espresso powder (if using). Stir in the cooled chocolate mixture. With a rubber spatula, fold in the flour and salt until just combined, then fold in the espresso beans.

Spread the batter into an even layer in the prepared pan and bake 25 to 30 minutes, until a cake tester inserted into the center comes out clean. Cool completely on a wire rack before frosting.

Place the butter in the bowl of a stand mixer fitted with the paddle attachment and mix until smooth. Add remaining ingredients and beat on low speed until combined. Turn mixer to high and beat until very fluffy, 2 to 3 minutes. (If frosting is too thick, add additional Irish cream or milk to make it spreadable.) Spread over cooled brownies and top with additional chocolate covered espresso beans, if desired. Slice into bars and serve.