Whisk the dry ingredients: In a medium bowl, whisk together the flour, bicarbonate of soda, baking powder, and salt. Watch for the flour to become light and aerated, with no lumps remaining—it should feel like fine sand when you lift the whisk.

Cream the butter and sugars: In a large mixing bowl, combine the melted butter, light brown sugar, and granulated sugar. Beat vigorously with a wooden spoon or hand mixer for 2-3 minutes. The mixture will transform from a gritty, separated paste into a smooth, glossy paste that smells deeply of caramelised sugar. It should hold a soft ribbon when you lift the spoon.

Add the eggs and vanilla: Beat in the whole egg, then the egg yolk, followed by the vanilla extract. Listen for the sound to change from a thick slosh to a silken, liquid rhythm. The batter will lighten in colour to a pale gold, and the texture will feel like thick custard.

Fold in the dry ingredients: Add the flour mixture in two additions, folding gently with a spatula. Stop as soon as the last streak of flour disappears—overworking will develop gluten, making the cookies tough. The dough will be soft, pliable, and slightly sticky to the touch.

Add the sprinkles: Gently fold the rainbow sprinkles into the dough with just a few turns. You want to see distinct, un-melted stripes of colour throughout the dough, not a murky grey mess. The dough should look like confetti-studded playdough.

Chill the dough: Cover the bowl with cling film and refrigerate for at least 30 minutes, or up to 72 hours. After chilling, the dough will be firm and no longer sticky to the touch. This rest allows the flour to fully hydrate and the fats to solidify, preventing the cookies from spreading into flat discs.

Preheat and portion: Preheat your oven to 180°C (160°C fan-forced) and line two baking trays with parchment paper. Scoop tablespoon-sized balls of dough and roll them between your palms until smooth. The balls should be about the size of a walnut, and they should feel cool and firm in your hands.

Bake: Place the dough balls 5cm apart on the trays. Bake for 12-14 minutes, rotating the tray halfway through. The edges will turn a deep golden brown and look set, while the centres will appear slightly puffed and pale—almost underdone. A faint crackling sound will emerge as the tops begin to crack. They will not look fully baked, and that is exactly right.

Cool and set: Let the cookies rest on the baking tray for 5 minutes. During this time, you will see the centres continue to bake from the residual heat, and the edges will firm up. After 5 minutes, transfer them to a wire cooling rack. They should feel soft but sturdy, with a slight bounce when pressed.