Prepare the Base: Line a 20cm x 20cm square baking tin with baking paper, leaving an overhang on two sides. Pulse the Biscoff biscuits in a food processor until they resemble fine, dark sand. Pour in the melted butter and pulse again until the mixture clumps together when pinched. Tip the mixture into the tin and press it down firmly and evenly with the base of a glass. The base should feel solid and compact, with a uniform dark brown colour. Pop it in the fridge to set while you make the filling.

Make the Filling: In a large bowl, beat the softened butter and Biscoff spread together with an electric mixer until they are pale and fluffy. The colour will lighten to a soft tan, and the mixture will look aerated. Add the light brown sugar and beat for another two minutes until the sugar has dissolved and the mixture feels gritty but smooth. Add the eggs one at a time, beating well after each addition. The batter will look glossy and thick. Stir in the vanilla bean paste.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, baking powder, and salt. Fold this into the wet mixture using a spatula until just combined. Do not overmix; you want to see no streaks of flour, but the batter should still be thick and slightly stiff. The smell at this stage is deeply aromatic, like toasted biscuits and vanilla.

Bake the Base Layer: Preheat your oven to 170°C (fan-assisted). Pour the filling over the chilled biscuit base and spread it into an even layer. Bake for 22-25 minutes. The top should be a deep golden brown, and a skewer inserted into the centre should come out with a few moist crumbs attached, not wet batter. The edges will have pulled away slightly from the tin. Let the bars cool completely in the tin on a wire rack.

Create the Caramel: While the bars cool, make the caramel. In a medium saucepan over a low heat, combine the butter, light brown sugar, and golden syrup. Stir continuously with a wooden spoon until the butter has melted and the sugar has dissolved. The mixture will look thick and grainy at first, then smooth and shiny. Turn the heat up to medium and let it bubble for 3-4 minutes, stirring constantly. It will darken to a rich amber colour and smell intensely of butterscotch. Remove from the heat and carefully whisk in the double cream. The sauce will bubble up violently, so take care. Whisk until smooth and glossy.

Assemble and Chill: Pour the hot caramel over the completely cooled Biscoff bar base. Tilt the tin gently to let the caramel run into the corners. Let it sit at room temperature for 10 minutes, then transfer to the fridge for at least 2 hours, or until the caramel is firm to the touch. The caramel should be set but still have a slight give when pressed.

Plate and Serve: Lift the bars out of the tin using the paper overhang. Place them on a cutting board. Using a sharp, long knife, slice the block into 16 even squares. Wipe the knife clean between cuts for the cleanest edges. Place each bar on a small dessert plate and finish with a delicate sprinkle of flaky sea salt. The contrast of the dark caramel, the pale salt, and the dense bar is visually stunning.