Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Grease a 20cm (8-inch) square baking tin and line it with baking parchment, leaving some overhang on the sides to act as handles for easy removal later.

Melt for the Brownie Base: In a medium, heatproof bowl set over a saucepan of simmering water (a bain-marie), melt the 115g of butter and 150g of dark chocolate together. Stir occasionally until smooth and glossy. Remove from the heat and set aside to cool slightly for about 5 minutes.

Mix the Brownie Batter: Whisk the 200g of caster sugar into the slightly cooled chocolate mixture. Add the 2 large eggs, one at a time, followed by the vanilla extract, whisking well after each addition until the batter is thick and shiny.

Combine Dry Brownie Ingredients: In a separate bowl, sift together the 65g plain flour, 25g cocoa powder, and ¼ tsp salt. Gently fold the dry ingredients into the wet chocolate mixture until just combined. It's important not to overmix here; stop as soon as you no longer see streaks of flour. Pour the batter into your prepared tin and spread it into an even layer.

Create the Cookie Dough: Now for the top layer. In a large bowl, using a stand mixer or electric hand mixer, cream the 115g of softened butter with the 100g light brown sugar and 50g caster sugar until light and fluffy, about 2-3 minutes. Beat in the 1 large egg and 1 tsp vanilla extract until well combined.

Finish the Cookie Dough: In another bowl, whisk together the 160g plain flour, ½ tsp bicarbonate of soda, and ¼ tsp salt. Gradually add the dry ingredients to the wet mixture, mixing on a low speed until just combined. Finally, stir in the 150g of chocolate chips.

Assemble the Brookies: Dollop spoonfuls of the cookie dough evenly over the brownie batter in the tin. You don't need to spread it perfectly; it will melt and spread as it bakes, creating a lovely cobbled effect. I find that flattening the dollops slightly with my fingers helps create a more even top.

Bake to Perfection: Place the tin in the preheated oven and bake for 30-35 minutes. You'll know it's ready when the cookie layer is golden brown and a wooden skewer inserted into the centre comes out with moist, fudgy crumbs attached, but not wet batter.

Cool Completely: This is the hardest part! Let the brookies cool completely in the tin on a wire rack. This allows the brownie layer to set properly. If you cut them while warm, they will be gooey and fall apart (though still delicious!). Once cool, use the parchment paper handles to lift the whole block out of the tin and cut into 16 squares.