Heat the oven to 175 C / 350 F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides.

Crush the speculoos biscuits in a food processor until fine crumbs form, or seal them in a zip-lock bag and crush with a rolling pin.

Mix the crushed biscuits with 80 g melted butter and 1 tbsp brown sugar until the crumbs look like damp sand.

Press the mixture firmly and evenly into the base of the lined pan using the back of a spoon.

Bake the crust for 8 minutes until lightly set, then remove and set aside while you make the batter.

Whisk 115 g melted butter and 150 g light brown sugar together in a large bowl until combined.

Add the eggs one at a time, whisking after each addition until the mixture looks smooth and slightly paler.

Stir in the vanilla extract.

Add the flour, baking powder, cinnamon, cloves, cardamom, nutmeg, white pepper, and salt. Fold with a rubber spatula until just combined, no dry streaks remaining.

Pour the batter over the pre-baked crust and spread into an even layer with a spatula.

Sprinkle 1 tbsp light brown sugar evenly over the top.

Bake for 22 to 25 minutes until the top is set and a toothpick inserted in the center comes out with a few moist crumbs.

Cool the bars completely in the pan on a wire rack, at least 30 minutes, before lifting out using the parchment overhang.

Slice into 12 bars with a sharp knife, wiping the blade between cuts for clean edges.