Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper, and set aside.

Place the butter, sugar, cocoa powder, and salt into a microwave-safe bowl, and heat for 30 seconds. Stir well, then continue heating in 30 second intervals (stirring after each one) until combined and very warm/hot to the touch.

Add the eggs and vanilla, and mix well until combined. Fold in the flour until fully combined, then gently fold in the chocolate chips. Be sure not to let them ALL melt into the warm batter.

Pour half of the batter into the prepared pan, and spread out evenly. Place the whole Oreos evenly over the batter, and top with the remaining batter. Top with the chopped Oreos.

Bake for 30 - 35 minutes, or until the centre is no longer jiggly and a toothpick inserted comes out almost clean with a few moist crumbs.

Allow to cool completely in the pan on a wire rack before cutting into squares.