Preheat the oven to 350°F (175°C) and grease a baking pan.

In a mixing bowl, whisk together the sugar and melted butter until combined.

Add the eggs, vanilla extract, and Irish cream liqueur, and mix until smooth.

Sift in the flour, cocoa powder, and salt, and stir until just combined. Fold in chocolate chips if using.

Pour the batter into the prepared baking pan and spread evenly.

Bake for 25-30 minutes, or until a toothpick inserted comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan.

In a mixing bowl, beat together the powdered sugar, softened butter, Irish cream liqueur, and vanilla extract until smooth and creamy.

Spread the frosting over the cooled brownies.

Cut into squares and serve.