Preheat oven to 350 degrees F.

Melt butter in a small saucepan over medium heat; remove pan from heat and stir in chocolate chips and espresso powder until melted and well combined.

In a small mixing bowl, using a mixer on medium speed, beat together cream cheese and Bailey's until well combined.

In a medium sized mixing bowl, sift together flour, cocoa powder, baking powder and salt.

In a large mixing bowl, whisk together egg and granulated sugar until well combined.

Whisk in melted chocolate mixture and cream cheese mixture until well combined.

Stir in flour mixture until just combined; working in batches using a mini ice cream scoop, divide batter into 34 mini muffin liners that are placed on sheet pans.

Bake for 15-17 minutes or until puffed and set; cool completely.

To make the frosting:

In a medium sized mixing bowl, using a mixer on medium speed, beat together butter, confectioners' sugar, Bailey's and vanilla extract until well combined and creamy - about 1-2 minutes.

Pipe or schmear frosting onto brownie cups.