Preheat oven to 350 degrees.
Using a hand mixer or a stand mixer fitted with the whisk attachment, combine melted butter, sugar, eggs and vanilla extract. Mix until combined.
Add in dry ingredients (cocoa powder, flour, baking powder, salt). Mix until combined.
Add melted chocolate. Mix until combined.
Line an 8x8 or 9x9-inch pan with aluminum foil or parchment paper.
Pour the batter into the pan and spread evenly. Bake for 20-25 minutes or until a toothpick comes out clean.
Remove from oven and set aside to cool.