Prepare the brownie batter: In a large mixing bowl, whisk the melted butter and caster sugar together until they form a smooth, glossy paste. The mixture will look thick and will smell deeply of butter. Add the eggs one at a time, whisking vigorously after each until the batter becomes silken and slightly thickened. Stir in the vanilla.

Add the dry brownie ingredients: Sift the cocoa powder, flour, and salt directly into the wet mixture. Fold gently with a spatula until just combined—stop as soon as you see no more streaks of flour. The batter will be thick, dark, and intensely fragrant, like the inside of a chocolate box. Stir in the dark chocolate chips if using. Set aside.

Make the cookie dough: In a separate bowl (or the bowl of a stand mixer), beat the softened butter, light brown sugar, and caster sugar until pale and fluffy. This will take about 3–4 minutes by hand or 2 minutes with a mixer. The mixture should look light and airy, almost like a cloud. Beat in the egg and vanilla until combined.

Finish the cookie dough: In a small bowl, whisk together the flour, bicarbonate of soda, and salt. Add this to the wet cookie mixture in two additions, mixing on low speed (or by hand) until a soft, slightly sticky dough forms. The dough should hold together when pressed but still feel tender. Fold in the milk chocolate chips with a spatula.

Layer the batters: Preheat your oven to 180°C (fan-assisted). Line a 23cm x 33cm baking tray with parchment paper, leaving an overhang for easy lifting. Spread the brownie batter evenly into the prepared tray. It will be thick, so use a spatula to coax it into the corners. Then, dollop the cookie dough in spoonfuls over the top. Don’t worry about spreading it perfectly—you want uneven patches.

Create the marbled effect: Using a butter knife or a skewer, gently swirl the two batters together. Drag the knife through the cookie dough down into the brownie, then lift and zigzag across the tray. Stop after 8–10 swirls—you want distinct ribbons of each batter, not a fully combined mess. The surface should look like a painter’s palette, with dark brown and golden patches.

Bake until set: Place the tray in the preheated oven and bake for 35–40 minutes. The top should be lightly golden and feel firm to the touch, but a skewer inserted into the centre should come out with a few moist crumbs attached (not wet batter). The edges will pull away slightly from the parchment, and the kitchen will smell like a bakery. Let the tray cool completely on a wire rack before slicing—this is crucial for clean squares.