Preheat your oven to 180°C (350°F) and line two baking trays with parchment paper. The paper should lie flat against the tray with no curled edges — this ensures even browning and easy removal later.

In a large mixing bowl, whisk together the flour, caster sugar, baking powder, bicarbonate of soda, and salt. The mixture should look pale and uniform, with no lumps of sugar or flour visible. You'll smell the dry, slightly sweet aroma of the baking powder.

Add the cold butter cubes to the dry ingredients. Using your fingertips or a pastry cutter, rub the butter into the flour until the mixture resembles coarse breadcrumbs. You should see small, pea-sized pieces of butter throughout — these will create steam pockets as the cookies bake, giving you that tender, short texture. The mixture should feel cool and slightly greasy to the touch.

In a small jug, whisk together the double cream, egg, and vanilla extract until the egg is fully incorporated and the mixture looks pale yellow and slightly frothy on top. Pour this wet mixture into the butter-flour mixture and stir with a wooden spoon until just combined. The dough will look shaggy and slightly sticky — do not overmix, or the cookies will turn tough.

Gently fold in the diced strawberries using a silicone spatula. Fold only 4–5 times — you want the berries evenly distributed but not crushed into the dough. The dough should be soft, slightly tacky, and studded with red berry pieces. If it feels too wet to handle, pop it in the fridge for 10 minutes.

Scoop rounded tablespoons of dough (about 40g each) onto the prepared baking trays, spacing them 5cm apart. The dough balls should look rough and uneven on top — do not roll them smooth. Sprinkle the tops with demerara sugar if using, which will create a lovely crisp, sugary crust as they bake.

Bake for 12–14 minutes, rotating the trays halfway through. The cookies are ready when the edges are golden brown and the tops are set but still pale in the centre. They will look slightly puffed and feel soft to a gentle touch — they continue baking on the tray as they cool. You'll smell a sweet, buttery, vanilla fragrance filling your kitchen.

Remove from the oven and let the cookies rest on the baking tray for 5 minutes. During this time, they will settle and firm up slightly. Then transfer them to a wire cooling rack using a flat spatula. The bottoms should be golden and the centres soft and tender. Let them cool completely if serving later, or enjoy them warm while the strawberries are still soft and juicy.