Step 1

Position an oven rack in the middle of the oven and preheat to 350 degrees F. Coat an 8-by-8-inch baking pan with nonstick spray, line with parchment and spray again.

Step 2

Combine the melted butter, granulated sugar and confectioners’ sugar in a large bowl. Whisk until a thick paste forms, about 1 minute. Add the milk, vanilla extract, baking powder and salt and whisk to incorporate. Add the flour, cocoa powder and chocolate chips and fold with a rubber spatula just until combined, being careful not to overmix.

Step 3

Transfer the batter to the prepared baking pan and smooth into an even layer. Bake until fully set and the top is firm, 30 to 40 minutes.

Step 4

Let cool completely in the pan on a wire rack, about 1 hour. Slice into 9 squares and serve.