Prep the pan. Line a 9-inch springform pan with parchment on the base and lightly grease the sides. Preheat the oven to 325°F (163°C).
Make the brownie base.
In a heatproof bowl, melt butter and chocolate together (microwave in 20-second bursts or use a double boiler). Whisk in sugar until glossy.
Add eggs and vanilla. Whisk until smooth and slightly thickened, about 30 seconds.
Fold in dry ingredients.
Sift in flour, cocoa, and salt. Stir gently until just combined. Spread into the pan and level the top.
Bake the brownie.
Bake for 15–18 minutes until set at the edges and slightly soft in the center. It will bake further later. Cool 10 minutes while you make the cheesecake.
Mix the cheesecake batter.
Beat cream cheese and sugar on medium-low until smooth, scraping the bowl. Add sour cream, vanilla, warmed Biscoff, and salt. Mix just until combined.
Add eggs.
Beat in eggs one at a time on low until just incorporated. Do not overmix.
Layer and bake. Pour cheesecake batter over the warm brownie base.
Tap the pan gently to release bubbles. Bake at 325°F (163°C) for 35–45 minutes, until the edges are set and the center still has a slight wobble.
Cool gradually. Turn off the oven, crack the door, and let the cheesecake sit for 30 minutes.
Then cool at room temperature for 1 hour.
Chill. Cover and refrigerate for at least 6 hours, preferably overnight, to fully set.
Make the Biscoff topping. Gently warm Biscoff spread with 2–3 tbsp cream or milk until pourable but not hot.
Stir until smooth.
Finish. Unmold the chilled cake. Pour the Biscoff topping over the cheesecake layer, letting it settle into an even glaze.
Garnish with crushed Biscoff cookies if you like. Chill 15–20 minutes to set the top before slicing.