Prepare the Tin and Oven: Preheat your oven to 180°C (160°C fan). Lightly grease a 20cm x 20cm square baking tin and line it with baking parchment, leaving some overhang on two sides to act as handles for easy removal later.

Make the Shortbread Base: In a large bowl, add the plain flour, caster sugar, and salt. Add the cold, cubed butter. Using your fingertips, rub the butter into the flour mixture until it resembles fine breadcrumbs. You can also do this in a food processor by pulsing gently.

Bake the Base: Tip the crumbly mixture into your prepared tin. Press it down firmly and evenly using the back of a spoon or the base of a glass. Prick the base all over with a fork. Bake for 20-25 minutes, or until it's a pale golden colour and firm to the touch. Set aside to cool slightly while you prepare the fillings.

Create the Chocolate Ganache: Place the finely chopped dark chocolate in a heatproof bowl. Gently heat the double cream in a small saucepan until it just begins to simmer around the edges – don't let it boil. Pour the hot cream over the chocolate and let it sit, undisturbed, for 5 minutes. Then, stir gently with a spatula until the mixture is completely smooth, glossy, and combined. This gentle method helps prevent the ganache from splitting.

Make the Raspberry Topping: While the ganache sits, place the raspberries, caster sugar, and lemon juice in another small saucepan over a medium heat. Cook for 5-7 minutes, stirring occasionally, until the raspberries have broken down and the mixture is juicy. In a small bowl, mix the cornflour with 1 tablespoon of cold water to create a slurry. Pour this into the raspberry mixture and stir continuously. Bring to a simmer and cook for another 1-2 minutes until the sauce has visibly thickened. Remove from the heat.

Assemble the Layers: Pour the smooth chocolate ganache over the slightly cooled shortbread base. I find that using an offset spatula helps to spread it into an even layer without disturbing the base. Spoon dollops of the warm raspberry topping over the ganache.

Swirl and Bake Again: Use a skewer or the tip of a knife to gently swirl the raspberry topping through the chocolate. Be careful not to overmix; you want to create a marbled pattern, not a uniform pink colour. Return the tin to the oven and bake for another 15-20 minutes, until the filling is set around the edges but still has a slight wobble in the centre.

Chill Completely: This is the most important step! Allow the bars to cool completely in the tin at room temperature. Once cool, cover and transfer to the refrigerator to chill for at least 4 hours, or preferably overnight. This firms up the layers, deepens the flavour, and makes them possible to slice cleanly. Once chilled, use the parchment handles to lift the slab out of the tin and cut it into 16 squares.