Preheat the oven to 350°F (175°C). Grease a small baking dish or ramekin with butter or nonstick spray.
In a small mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Stir until smooth and well combined.
Add the cocoa powder, baking powder, and salt to the bowl. Mix until the dry ingredients are fully incorporated and no lumps remain.
Stir in the flour, vanilla extract, and milk. Mix until the batter is smooth and thick.
Fold in the chocolate chips until evenly distributed throughout the batter.
Pour the batter into the prepared baking dish and spread it evenly with a spoon or spatula. Bake in the preheated oven for 12 to 15 minutes, or until the edges are set and the center is slightly soft to the touch.
Remove from the oven and let cool for 5 minutes before serving.