Preheat oven to 350° and line a 9x13 inch pan with parchment paper, then spray with oil or just spray pan with oil.
Melt butter in medium saucepan, remove from heat. Add sugar, eggs, vanilla, cocoa powder, flour, salt and baking powder, stirring after each addition. Mix in espresso powder until just combined. Gently stir in chocolate chips.
Pour into prepared pan and bake for 25-30 minutes. DO NOT OVERBAKE. Brownies will settle and firm up as they cool. Cool completely in pan on cooling rack.
Once cooled, I find it easiest to freeze (up to several weeks if properly wrapped) take out of freezer for 20 minutes then cut brownies, they are a bit easier to handle when slightly frozen.
Using a small 1-1 ½ inch round cookie cutter cut as many brownie bites as possible, getting as close to edges and previous cut as possible. Should yield about 30 bites. Frost.
KAHLUA BUTTERCREAM
Add butter to a medium bowl and beat with an electric mixer until smooth. Add cream and mix to combine. In a small bowl, combine Kahlua and espresso powder, stirring briskly with a spoon to dissolve the espresso powder. Add Kahlua mixture to butter and mix again.
With mixer on low speed, add powdered sugar, one cup at a time, mixing until smooth after each addition. Beat on medium high, scraping sides once or twice until light and fluffy. Spoon buttercream into a pastry bag fitted with a large plain tip (I used Wilton #12). Add a little "kiss" of buttercream to the top of each brownie bite.
Store in airtight container in fridge or freezer, bringing to room temp prior to serving.
Note: If you don't want to pipe the buttercream, you could simply spread some buttercream onto the top of each brownie bite or on the entire pan of brownies and cut as usual.
Note 2: Don't like espresso or Kahlua? Just omit it, these would also be delicious with Bailey's in the frosting!