Preheat your oven to 350°F (175°C) and grease a mini muffin tin or line it with mini cupcake liners.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.
In another bowl, mix the melted butter, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. If using, fold in the chocolate chips and/or chopped nuts.
Spoon the batter into the prepared mini muffin tin, filling each cup about 2/3 full.
Bake in the preheated oven for 10-12 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownie bites to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.