Preheat your oven to 350°F (175°C) and grease an 8x12-inch baking pan, then line it with parchment paper.

In a medium bowl, mix the melted butter and vegetable oil. Stir in the white and brown sugars until glossy. Add the eggs and vanilla, whisking until fluffy.

Sift the flour, cocoa powder, and salt into the wet mixture. Gently fold without overmixing.

Stir in the chocolate chips, then pour the batter into the prepared pan, spreading evenly and topping with extra chocolate.

Bake for 25-30 minutes until set around the edges; the center may jiggle

Let cool in the pan for 15-20 minutes before lifting out and cooling completely on a wire rack. Cut into 16 squares.