Prepare the Biscoff centers:
Scoop out 1-teaspoon portions of Biscoff spread onto parchment paper. Freeze for at least 1 hour until solid.

Brown the butter:
In a saucepan, melt butter and cook until golden and nutty (about 5–7 minutes). Let it cool slightly.

Make the dough:
In a large bowl, whisk browned butter, brown sugar, and granulated sugar. Add eggs and vanilla. Stir in flour, baking soda, and salt until just combined. Fold in white chocolate.

Stuff the cookies:
Scoop 3-tablespoon-sized dough balls (about 90g each), flatten slightly, place a frozen Biscoff center in the middle, and seal tightly. Chill dough balls for at least 30 minutes or overnight.

Bake:
Preheat oven to 350°F (175°C). Place chilled cookie balls on a parchment-lined tray. Bake for 10–13 minutes, or until edges are golden and centers slightly underbaked.

Cool & enjoy:
Let cookies cool on the pan for 5 minutes before transferring to a rack. Sprinkle with flaky salt if desired.