Preheat oven to 350 F. Grease a 24-cup mini muffin tin with butter. Set aside.
In a medium saucepan, melt butter over low heat. Whisk in granulated sugar, cocoa powder and salt directly in the pot. Remove from heat and let cool slightly.
Whisk in the eggs and vanilla until combined.
Stir in the flour and mini chocolate chips. Do not over-mix.
Scoop batter into grease mini muffin tin, filling each cup about three-quarters full (about 2 tablespoons). Bake at 350 F for 10 to 12 minutes, or until edges are set and centers are soft. Slightly underbaking them will yield a fudgier brownie.
Let the brownies cool in the pan for 5 minutes before running a butter knife around the edges to release the brownie bites. Transfer to a wire rack to cool completely.
Notes
Muffin pan: In order to get the muffins out of the pan, use a nonstick pan and grease each cup with nonstick spray before adding the batter.Store in an airtight container at room temperature for 5 days or in the refrigerator for a week.Freeze in an airtight container for up to 3 months. Wrap in plastic wrap to keep them extra fresh.