Make the base. Line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang for easy removal. Blitz the digestive biscuits in a food processor until they resemble fine, damp sand. Pour in the melted butter and pulse until the mixture clumps together when pressed. Tip the crumbs into the tin and press them down firmly with the back of a spoon, ensuring an even, compact layer. The base should feel dense and solid, like wet sand at the beach. Bake at 180°C (160°C fan) for 10 minutes, until the edges just begin to darken and the kitchen smells toasty. Set aside to cool.

Start the caramel. In a medium, heavy-bottomed saucepan, combine the condensed milk, brown sugar, cubed butter, and golden syrup. Place the pan over a low heat and stir gently with a wooden spoon. Do not rush this stage. You want the butter to melt slowly and the sugar to dissolve completely. The mixture will look grainy at first, but as it warms, it will become a smooth, pale golden liquid. Listen for the gentle hiss of the heat against the pan.

Simmer the caramel to perfection. Once the mixture is smooth, increase the heat to medium and bring it to a gentle, lazy bubble. Continue to stir frequently, scraping the bottom and sides of the pan to prevent scorching. Let it bubble for 5-7 minutes. The colour will deepen to a rich amber, and the texture will thicken noticeably, leaving a trail on the bottom of the pan when you stir. It should smell deeply of toasted sugar and butter. Remove from the heat and stir in the vanilla extract and the Biscoff spread until the caramel is glossy and fully combined.

Pour and chill. Pour the hot caramel over the cooled biscuit base. Tilt the tin gently to spread it evenly. Tap the tin on the counter a few times to release any air bubbles. Place the tin in the refrigerator for at least 1 hour, or until the caramel is firm to the touch and no longer jiggles. The surface should look set and matte.

Make the topping. In a heatproof bowl set over a pan of barely simmering water (make sure the bowl doesn't touch the water), combine the double cream and chopped white chocolate. Stir gently until the chocolate has melted and the mixture is smooth and silky. The texture should be thick enough to coat the back of a spoon. Remove from the heat and let it cool for 5 minutes, stirring occasionally, until it has a thick, pourable consistency.

Assemble and set. Pour the white chocolate topping over the chilled caramel layer, spreading it right to the edges with a small offset spatula. While the topping is still wet, sprinkle the crushed Biscoff biscuits and a generous pinch of sea salt flakes over the surface. Return the tin to the refrigerator for another 2 hours, or until the topping is completely firm and set. When you gently press the surface, it should feel firm and not stick to your finger.

Slice and serve. Lift the block out of the tin using the parchment overhang. Place it on a cutting board and use a sharp, long knife to slice it into 16 even bars. For the cleanest cuts, wipe the knife blade clean between each slice. The base should hold together firmly, the caramel should be chewy but not sticky, and the topping should be creamy and smooth.