Preheat the oven to 350 degrees F (175 degrees C). Line a mini muffin tin with plain or seasonally colored paper liners.
Combine butter, sugar, eggs, and vanilla extract in a large bowl. Mix vigorously with a wire whisk until butter is incorporated but you can still see bubbles of butter in the batter. Add flour, cocoa powder, baking powder, and salt. Mix gently until just incorporated; do not overmix.
Divide batter among the muffin cups using a small ice cream scoop. Gently cover the top of the brownies with a dusting of sea salt flakes.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 10 to 15 minutes. Transfer to wire racks to cool.
