Preheat oven to 350 degrees and line a loaf pan with parchment paper or aluminum foil.
Melt butter in a small dish in the microwave and set aside to cool slightly.
In a medium bowl, whisk together the flour, sugar, cocoa powder, sea salt, and baking powder.
Add egg and vanilla and stir.
Add melted butter and mix until smooth.
Fold in melted chocolate and stir until just combined.
Pour batter into a prepared loaf pan and bake for 20-22 minutes, rotating halfway through. Brownies will puff up slightly as they bake, but the top will fall when they are done.
Allow brownies to cool in the pan for 15-30 minutes. Lift out of the pan, slice, and serve.
Store brownies in an airtight container for 3-4 days or freeze.