Preheat your oven to 180°C (160°C fan) and line two baking trays with parchment paper. The paper should feel smooth and flat against the tray — any wrinkles will make your cookies spread unevenly.

Cream the butter and sugars together in a large mixing bowl using a stand mixer or hand mixer on medium speed. Beat for 2-3 minutes until the mixture looks pale, fluffy, and slightly lighter in colour — it should smell sweet and buttery, and feel soft when you press it with a spoon.

Add the egg and vanilla and beat on low until just combined. The mixture will look slightly glossy and smell wonderfully of vanilla. Scrape down the sides of the bowl with a spatula — you should see no streaks of butter remaining.

Whisk the flour, bicarbonate of soda, and salt together in a separate small bowl, then gradually add them to the wet mixture. Mix on low speed until you see a few streaks of flour remaining — stop here; overmixing makes cookies tough. The dough will feel soft and slightly sticky to the touch.

Fold in the M&Ms and chocolate chips using a spatula. You'll hear a gentle clinking sound as the sweets knock against the bowl, and the dough will become studded with bright colours. Reserve about 2 tablespoons of M&Ms for the tops.

Scoop rounded tablespoons of dough (about 30g each) onto the prepared baking trays, leaving 5cm between each. The dough balls should look slightly lumpy and uneven on top — that's good, it gives the cookies a homemade appearance.

Press a few reserved M&Ms gently into the top of each dough ball. This ensures every cookie has visible, glossy sweets on its surface after baking.

Bake for 10-12 minutes. At 10 minutes, the edges should look golden brown and set, while the centres appear pale, puffy, and slightly underdone. The kitchen will smell like a bakery — warm vanilla and toasted sugar.

Cool on the tray for 5 minutes — the cookies will continue to set as they rest. You'll see the centres sink slightly and the edges firm up. Transfer to a wire cooling rack using a spatula and leave until completely cool. The cookies should feel slightly crisp around the edges and soft in the middle when you pick one up.