Prepare the Brownie Batter: Preheat your oven to 180°C (160°C fan) and line a 20cm x 20cm square baking tin with parchment paper, leaving some overhang for easy lifting. In a large bowl, whisk together the melted butter and caster sugar until the mixture looks glossy and smooth. Add the eggs and vanilla extract, whisking vigorously until the batter is thick and pale, and you can see a ribbon-like trail when you lift the whisk. Sift in the cocoa powder, flour, and salt. Fold gently with a spatula until just combined—the batter will be thick and smell intensely of rich chocolate. Spread this evenly into the prepared tin.

Make the Cookie Dough: In a separate large bowl, beat the softened butter, light brown sugar, and caster sugar together with a hand mixer until the mixture is light, fluffy, and pale in colour—this takes about 3 minutes. Beat in the egg and vanilla extract until fully incorporated, scraping down the sides of the bowl. In a small bowl, whisk the flour, bicarbonate of soda, and salt together. Add the dry ingredients to the wet mixture and mix on low speed until a soft dough forms and no dry flour remains. Fold in the chocolate chips with a spatula. The dough should be soft and hold its shape slightly.

Layer the Cookie Dough: Drop spoonfuls of the cookie dough over the brownie batter. Don't worry about being neat—use your fingers or the back of a spoon to gently press the dough into an even layer, covering the brownie batter completely. It will seem like a lot of dough, but it bakes down beautifully. The top should look like an unfinished cookie base, with visible chocolate chips dotting the surface.

Bake to Perfection: Place the tin in the preheated oven and bake for 30-35 minutes. At the 25-minute mark, check the colour—the edges should be a deep golden brown and the centre should look set but still slightly soft. A toothpick inserted into the cookie layer should come out with a few moist crumbs, not wet batter. The brownie layer will be fudgy, so the toothpick test works best on the top cookie layer. The bars will puff up slightly and the kitchen will smell like a bakery.

Cool Completely: Remove the tin from the oven and place it on a wire cooling rack. Let the brookies cool completely in the tin—this is essential. As they cool, the layers will set and the bars will become easier to slice. The surface will settle and become flat. Once fully cool (about 2 hours), use the parchment paper overhang to lift the entire slab onto a chopping board. Use a large, sharp knife to cut into 16 even squares. The brownie layer will be dark and moist, while the cookie layer will be golden with melted chocolate pockets.