Preheat oven to 350°F. Grease a 12-cup muffin tin with cooking spray or butter.
Prepare brownie batter according to packaging instructions. Stir in espresso powder and Bailey's Irish Cream Liqueur.
Add batter to muffin tin. Bake for 25 minutes.
When you remove the brownies from the oven, immediately press the bottom of a shot glass in the center of each to form them into cups.
Let brownies cool completely.
While the brownies are cooling, add butter for the frosting to the bowl of a stand mixer. Add in Bailey's Irish Cream Liqueur.
Beat until combined completely.
Add powdered sugar. Beat on low until sugar is incorporated. Increase speed to medium-high and beat until fluffy, making sure to scrape down the sides every so often.
Pipe filling into cooled brownie cups. Top with St. Patrick's Day themed sprinkles, if desired.