Preheat the oven to 350ºF and line an 8-inch square baking dish with parchment paper. In a small bowl, whisk together the flour, salt, and baking soda; set aside.

In a medium saucepan over medium heat, melt the butter. Remove from the heat and stir in the cocoa powder until smooth and glossy. Whisk in both sugars. Add the eggs and vanilla, stirring until fully incorporated. Switch to a wooden spoon and stir in the dry ingredients.

Pour batter into the prepared baking dish and bake for 25-28 minutes. Remove from the oven and allow to cool completely.

Using a stand mixer fitted with a paddle attachment, whip the butter until smooth and creamy. Add the powdered sugar and continue whipping, stopping and scraping down sides of bowl as needed. Add the Irish cream, and continue whipping until light and fluffy, about 1 minute. Spread evenly onto the cooled brownies.

Sprinkle toffee bits on top of the brownies. Cut into squares. Serve and enjoy!