Crush cookies: Place 100g Biscoff cookies in sealed bag. Crush with rolling pin until fine crumbs + small chunks. Crush 30g separately for topping.
Melt butter: Melt 90g butter in pan or microwave. If browning, cook until golden with brown specks. Pour into large bowl. Cool 5 minutes.
Mix wet: Add 80g Biscoff spread to warm butter. Whisk until smooth. Add brown sugar and granulated sugar. Whisk 1 minute. Add egg and vanilla. Whisk another minute until glossy and slightly thick.
Add dry: Add flour, baking soda, and salt. Stir with spatula until just combined. Few flour streaks fine.
Fold: Add semi-sweet chocolate chips, milk chocolate chips, and 100g crushed Biscoff cookies. Fold until evenly distributed.
Chill: Cover bowl. Refrigerate minimum 1 hour (up to 24 hours).
Preheat and prep: When ready, preheat oven to 175°C (350°F). Line baking sheet with parchment.
Scoop and bake: Scoop 35g dough balls (1.5 Tbsp). Place 5cm apart. Bake 10-12 minutes until edges golden, centers soft.
Add topping: Remove from oven. Immediately press reserved 30g crushed Biscoff onto warm cookies. Rest on sheet 5 minutes, then transfer to wire rack.
Drizzle: Melt 40g Biscoff spread in microwave 20-30 seconds. Drizzle over fully cooled cookies. Let set 20 minutes.