Preheat oven to 350°F. Line a 9-inch x 13-inch baking pan with parchment paper, leaving an overhang on both sides. Grease the parchment and set aside.
Melt butter in a medium saucepan set over medium heat. Remove from heat and add granulated sugar, brown sugar, cocoa powder and salt. Whisk together until combined.
Whisk in vanilla. Add the eggs one at a time, whisking after each addition until smooth and shiny. Add flour and stir until just mixed. Pour batter into prepared pan and smooth the surface.
Bake 30-35 minutes, until a toothpick comes out mostly clean and the edges are set. Let cool at least 1 hour.
Use the parchment paper sling to carefully remove the brownies from the pan. Use a small round cookie cutter (1 ½-inch) to cut out circle bites. Decorate with mint buttercream frosting.
Pull butter out of the fridge and let it soften for 10 minutes.
Place butter in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed to soften, about 30 seconds. Add all of the sifted powdered sugar, attach splash guard, and mix on low speed until it just comes together, about 3 minutes. Mixture will look very powdery at first. Scrape down sides and bottom of bowl, and mix again.
Turn mixer on high, immediately add mint extract and hot water, and mix until just combined, about 20 seconds. If desired, mix in a few drops of green food coloring. Transfer to a piping bag and decorate brownie bites as desired.