Prep the pan: Heat oven to 350°F (175°C). Line an 8-inch (20 cm) square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.
Melt chocolate and butter: In a medium saucepan over low heat, melt the butter and chopped chocolate, stirring until smooth.
Remove from heat and let cool 3–4 minutes so it’s warm, not hot.
Whisk in sugars: Add granulated and brown sugar to the warm chocolate mixture. Whisk until glossy and combined.
Add eggs and flavor: Whisk in eggs one at a time until the batter looks thick and shiny. Stir in vanilla and 1/4 cup Irish cream.
Fold in dry ingredients: Sift flour, cocoa, salt, and espresso powder into the pot.
Fold gently with a spatula just until no dry streaks remain. If using, fold in chocolate chips.
Bake: Spread batter evenly in the prepared pan. Bake 22–28 minutes, until the top is set and a toothpick inserted near the center comes out with a few moist crumbs.
Do not overbake.
Cool completely: Set the pan on a rack and cool fully, about 1–1.5 hours. Lift out using parchment and transfer to a board.
Make the glaze: In a bowl, whisk powdered sugar, melted butter, a pinch of salt, and 2 tbsp Irish cream. Add up to 1 more tablespoon Irish cream to reach a slow-drizzle consistency—thick but pourable.
Glaze and set: Spread glaze over cooled brownies.
Let it set 20–30 minutes before slicing into 16 squares.