Preheat the oven to 350 degrees F. Spray an 8-inch square baking pan with cooking spray.
Microwave the chocolate in a small microwave-safe bowl in 30-second intervals, stirring in between each, until fully melted, 1 to 2 minutes. Set aside.
Beat the sugar, butter and eggs in a stand mixer fitted with the paddle attachment on medium-high speed until pale and fluffy, about 2 minutes. Whisk together the flour, salt and baking powder in a medium bowl. On low speed, slowly add the flour mixture to the butter mixture and mix until combined. Slowly add the melted chocolate, milk and coffee and mix until fully incorporated. Pour into the prepared baking pan and smooth the top with a rubber spatula.
Bake until a toothpick comes out clean, about 30 minutes. Let cool completely in the pan, then cut into 16 squares.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)