Preheat oven to 350°F (175°C). Line 8x8-inch pan with parchment. Mix 1 1/2 cups Biscoff crumbs with 1/4 cup melted butter; press into pan. Bake 8 minutes, cool slightly.
Whip 2 eggs, 1/2 cup granulated sugar, and 1/2 cup brown sugar on high 2 minutes until creamy.
Add 1/2 cup melted butter and 2 tsp vanilla; mix. Sift in 1/2 cup flour, 1/3 cup cocoa, 1/4 tsp baking powder, 1/4 tsp salt; fold gently.
Pour over crust. Dollop 1/3 cup warmed Biscoff spread; swirl with knife.
Bake 22-25 minutes until toothpick has moist crumbs. Cool fully, chill, slice.