Combine Dry Ingredients: In a medium bowl, whisk together the plain flour, bicarbonate of soda, and salt. This ensures the raising agent is evenly distributed. Set aside for later.

Cream Butter and Sugars: In a large bowl, using an electric hand mixer or a stand mixer with the paddle attachment, beat the softened unsalted butter with the light brown sugar and caster sugar on medium speed for about 3-4 minutes. You're looking for a mixture that's light, pale, and fluffy.

Add Wet Ingredients: Add the large egg and vanilla extract to the butter mixture and beat again until everything is well combined. Scrape down the sides of the bowl to ensure it's all incorporated.

Form the Dough: Tip the dry flour mixture into the wet ingredients. Mix on a low speed until just combined – be careful not to overmix. A few streaks of flour are fine. Gently fold in the dark chocolate chips with a spatula.

Chill the Dough: This is the most important step for chewy cookies that don't spread too much. Cover the bowl with cling film and place it in the refrigerator for at least 1 hour (or up to 3 days). Chilling solidifies the fat, which controls spread and deepens the flavour. You can learn more about the science behind this on food science sites like Serious Eats.

Bake the Cookies: Preheat your oven to 180°C (160°C fan). Line two large baking trays with baking parchment. Scoop out portions of dough (about 2 tablespoons each) and roll them into balls. Place them on the baking trays, leaving plenty of space between them as they will spread. You should get about 24 cookies.

Bake and Cool: Bake for 12-14 minutes, until the edges are golden brown but the centres still look slightly soft. What works best for me is rotating the trays halfway through for an even bake. Let them cool on the baking trays for 5 minutes before transferring them to a wire rack to cool completely. They must be 100% cold before assembling.

Assemble the Sandwiches: While the cookies cool, let the tub of ice cream sit at room temperature for 10-15 minutes to soften slightly. Match the cookies into 12 pairs of similar size. Turn one cookie of each pair upside down. Place a generous scoop of ice cream on it and gently press the second cookie on top.

Decorate and Freeze: If you're using decorations, pour your sprinkles or chopped nuts onto a plate. Gently roll the exposed ice cream edges of each sandwich in the sprinkles until coated.

Final Freeze: Wrap each ice cream sandwich individually and tightly in cling film or baking parchment. Place them on a flat tray in the freezer and freeze for at least 4 hours, or until completely firm.