Preheat the oven to 180°C (fan-assisted) and line two baking trays with parchment paper. The paper should feel taut against the tray — this prevents the cookies from spreading too much.

Cream the butter and sugars together in a large mixing bowl using a hand mixer or stand mixer on medium speed. Beat for 2–3 minutes until the mixture looks pale yellow and fluffy, and you can no longer see individual sugar grains. The texture should feel light and airy when you lift the beaters.

Add the egg and vanilla, then beat on low speed until fully combined. The mixture will look slightly glossy and will smell sweetly of vanilla. Scrape down the sides of the bowl with a spatula to ensure everything is evenly incorporated.

Fold in the dry ingredients — sift the flour, bicarbonate of soda, and salt directly over the butter mixture. Stir gently with a wooden spoon or spatula until just combined. You should see no streaks of white flour, but stop as soon as the dough comes together. Overmixing will make the cookies tough.

Stir in the chopped nuts until they are evenly distributed throughout the dough. The dough will feel slightly stiff and studded with crunchy pieces. Scoop rounded tablespoons of dough onto the prepared trays, leaving about 5cm of space between each ball — they will spread during baking.

Bake for 10–12 minutes, rotating the trays halfway through. The cookies are ready when the edges turn a deep golden brown and the centres look slightly puffed and pale. They will feel soft to the touch but should not appear wet or shiny. Let them cool on the trays for 5 minutes — they will continue to set — then transfer to a wire rack to cool completely.

Make the chocolate frosting while the cookies cool. Place the chopped dark chocolate, double cream, and extra butter in a heatproof bowl set over a pan of barely simmering water. Stir gently until smooth and glossy — the mixture should drip slowly from the spoon. Alternatively, microwave in 20-second bursts, stirring between each, until melted.

Frost the cookies once they are completely cool. Spoon about a teaspoon of frosting onto each cookie and spread gently with the back of the spoon. The frosting will set to a soft, fudgy texture as it cools. Let the frosted cookies sit for 10 minutes before serving.