Preheat the oven to 180C/350F/Gas 4. Line the bottom and sides of an 8x8" baking pan with aluminium foil, leaving an overhang on two opposite sides. Set aside.

Whisk together the butter, sugar, salt, and cocoa powder. Stir in the vanilla extract, then mix in the eggs, one at a time, stirring vigorously after each one. Fold in the flour until fully incorporated.

Pour the batter into the prepared pan, and spread out evenly. Bake for 20 - 25 minutes, or until a toothpick inserted into the centre comes out almost clean with a few moist crumbs. Allow to cool completely in the pan on a wire rack.

Using a handheld or stand mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy. Beat in the milk and vanilla extract. Add the flour and salt, and mix until combined. Fold in the chocolate chips.

Gently spread the cookie dough over the cooled brownies, and refrigerate whilst preparing the chocolate layer.

In the microwave, melt the butter and chocolate together, stirring every 30 seconds. Pour over the cookie dough, and spread out evenly. Refrigerate for 2 - 4 hours (or overnight) until firm. Cut into squares, and enjoy!