Preheat oven to 350°F (175°C) and grease or line an 8×8-inch pan.
Whisk melted butter and sugar, then add eggs one at a time. Stir in Baileys Irish Cream.
Sift flour, cocoa, and salt; fold into wet mixture until smooth. Add extra Baileys if batter is too stiff.
Pour batter into pan and bake 20–25 minutes until a toothpick shows moist crumbs. Cool completely.
Beat butter, gradually add powdered sugar, then Baileys to make frosting. Adjust consistency with cream if needed.
Spread frosting over cooled brownies, drizzle melted chocolate if desired, let set, then slice and serve.