Cream the Butter and Sugar: In the bowl of a stand mixer fitted with the paddle attachment, or using a hand mixer, beat the cold, cubed butter and the sifted icing sugar on a medium speed for about 2-3 minutes. Scrape down the sides of the bowl. It won't be light and fluffy like a cake batter; it should look like thick, creamy paste.

Add Vanilla: Beat in the vanilla extract until just combined.

Combine the Dry Ingredients: In a separate bowl, whisk together the plain flour, cornflour, and sea salt. Add this to the butter mixture all at once.

Form the Dough: Mix on the lowest speed until the flour is just incorporated and the mixture starts to form large, shaggy clumps. Be careful not to overmix. Tip the contents onto a clean work surface and gently bring it together with your hands into a smooth dough.

Shape and Indent: Roll the dough into a log about 5-6cm in diameter. Wrap it tightly in cling film and chill in the refrigerator for at least 30 minutes. Once chilled, unwrap the log and use a sharp knife to slice it into 1.5cm thick rounds. Place them on a baking tray lined with parchment paper, leaving about 5cm of space between each one. Use your thumb or the back of a rounded teaspoon to make a shallow indentation in the centre of each cookie. I find that a ½ teaspoon measure works best for a uniform well.

Prepare and Add the Swirl: Place the raspberry jam in a small, microwave-safe bowl and heat for 10-15 seconds, just until it's slightly loosened and easier to work with. Spoon a small amount (about ¼ teaspoon) of the warmed jam into the indentation of each cookie.

Create the Swirl: Take a toothpick or a small skewer and gently drag it through the jam and into the dough in a circular or figure-eight motion to create a marbled, swirled effect. Don't overdo it – two or three little swirls are enough.

The Final Chill: Place the entire baking tray of cookies into the refrigerator to chill for another 30 minutes. This step is crucial and ensures the cookies hold their shape perfectly during baking. While they chill, preheat your oven to 180°C (160°C Fan).

Bake to Perfection: Bake the chilled cookies for 12-15 minutes. The edges should be very lightly golden, but the tops should remain pale. They will feel slightly soft in the centre when you take them out.

Cool Completely: Let the cookies cool on the baking tray for 10 minutes before carefully transferring them to a wire rack to cool completely. They are delicate when warm but will firm up as they cool.