Preheat the oven to 350 degrees and line a mini muffin pan with paper liners.
Melt 1/2 cup butter in a microwave-safe bowl, then stir in the granulated sugar until combined.
Add the eggs and 1 teaspoon vanilla extract, mixing until smooth.
Stir in the cocoa powder, flour, and salt until no dry streaks remain.
Spoon the batter into the prepared mini muffin cups, filling each about two-thirds full.
Bake for 12 to 15 minutes until a toothpick comes out with a few moist crumbs.
Let the brownie bites cool completely in the pan.
Beat the remaining 1/2 cup butter with the powdered sugar, milk, and 1 teaspoon vanilla extract until creamy.
Add green food coloring a few drops at a time until the frosting reaches a bright green shade.
Spread or pipe the green frosting over each cooled brownie bite.