Preheat oven to 350°F (175°C). Grease a 9x9-inch baking pan with cooking spray and set aside.
Sift together the flour and salt; set aside.
Place the butter and chopped chocolate in a saucepan and melt over low heat, stirring frequently, until melted and smooth.
Remove from heat and stir the sugar into the chocolate mixture; let cool for 3 minutes.
Pour the chocolate mixture into your mixer's bowl and beat in the eggs, one at a time.
Mix in the 1 tablespoon of Baileys Irish Cream.
Stir in the flour mixture with a wooden spoon.
Spread the batter evenly into the prepared pan.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan on a wire rack.
Frosting Preparation and Serving
For the buttercream, place the softened butter, powdered sugar, and 2 tablespoons of Baileys Irish Cream in a large bowl and blend together with an electric mixer until smooth and creamy.
Spread the frosting evenly on top of the cooled brownies.
Cut into squares and serve.
Notes
These brownies are very fudgy. For a slightly cleaner cut, chill the frosted brownies before slicing.