Cream the Butter and Sugars: In a large bowl, beat the softened butter, caster sugar, and brown sugar together on medium speed for 3–4 minutes. You are looking for a pale, fluffy mixture that feels light and looks noticeably airy—the sound will shift from a heavy slap to a soft, smooth whisper.

Add the Eggs and Vanilla: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. The mixture will look glossy and slightly looser. Stir in the vanilla extract until the batter smells fragrant and sweet.

Whisk the Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and sea salt. This ensures the raising agent is evenly distributed—you don't want a pocket of soda creating a metallic taste in one single cookie.

Combine Wet and Dry: Gradually add the dry ingredients to the wet mixture, mixing on low speed. Stop just as the last streaks of flour disappear. The dough will be thick, sticky, and almost play-dough-like in consistency. Do not overmix; you should see no dry patches, but the dough should still feel soft and pliable.

Fold in the Sprinkles: Switch to a rubber spatula. Gently fold in the rainbow sprinkles and white chocolate chips (if using). You want the sprinkles suspended throughout the dough, not mashed into it. The dough should now look like a celebration—vivid specks of colour against the pale beige base.

Chill the Dough: Cover the bowl with cling film and refrigerate for at least 30 minutes, or up to 72 hours. This step is non-negotiable. Chilled dough spreads less, resulting in a thicker, chewier centre and a more defined dome. The dough will feel firm and cool to the touch when ready.

Preheat and Prepare: Preheat your oven to 180°C (160°C fan). Line two baking trays with parchment paper. Scoop the dough into balls roughly 45g each (about 1.5 tablespoons). Roll them between your palms until they are smooth spheres—this creates a uniform shape that bakes evenly.

Bake to Perfection: Place the dough balls on the trays, leaving at least 5cm between each to allow for spreading. Bake for 10–12 minutes. The edges will be a deep, golden brown and look set, while the centres will still appear pale, soft, and slightly puffed. You will smell the warm vanilla and caramelised butter long before the timer goes off.

Shape and Cool: Remove the tray from the oven. Immediately place a round cookie cutter (or a large glass) over a warm cookie and gently swirl it in a circular motion. This "scooting" technique pushes the edges inward, creating a perfectly round, uniformly thick cookie. Let the cookies cool on the tray for 5 minutes—they will continue to set from the residual heat—then transfer to a wire rack to cool completely.