Preheat your oven to 350°F (175°C). Line a 24-cup mini muffin tin with paper liners or grease generously with butter.
In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder until no lumps remain and the mixture is evenly colored.
In a large bowl, whisk the melted butter and sugar together until smooth and slightly glossy, about 1 minute. Add the eggs one at a time, whisking well after each addition until the mixture is thick and pale. Stir in the vanilla extract.
Add the dry ingredients to the wet ingredients and fold gently with a spatula just until no flour streaks remain. Do not overmix or the bites will be cakey instead of fudgy. Fold in chocolate chips if using.
Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Use a small cookie scoop for uniform sizing.
Bake for 12-15 minutes, until the edges are set and the tops look dry but the centers still jiggle slightly when gently shaken. The tops should have a light crackle.
Let the brownie bites cool in the tin for 10 minutes to firm up, then transfer to a wire rack to cool completely. They will continue to set as they cool and develop that perfect fudgy texture.