Preheat the oven to 180C/350F/Gas 4. Line a baking tray with parchment paper or a silicone mat, and set aside.

Whisk together 1 and 1/2 cups (187g) flour, baking powder, baking soda, cornflour, and salt. Set aside.

In a separate large mixing bowl, whisk together the melted butter and sugars until combined.

Add the egg and vanilla, and mix until combined.

Add the dry ingredients, and mix until just combined.

Transfer half of the dough to a separate medium mixing bowl.

To one half of the dough, mix in the remaining 1/4 cup (30g) flour, and half of the chocolate chips.

To the other half, mix in the cocoa powder and the remaining chocolate chips.

Roll both cookie dough into 1-oz balls (or use a small cookie scoop).

Roll two balls (one from each colour) together to form a larger ball. Tear apart with your fingers, then roll the two halves together once again to create a more marbled effect.

Place onto the prepared baking tray, and bake for 10 - 12 minutes.

Allow to cool on the baking tray for 5 - 10 minutes before transferring them to a wire rack to cool completely.