Preheat oven to 350F. Line an 8x8-inch metal pan with parchment, leaving overhang on two sides for easy removal. Lightly butter any exposed pan surface.

In a medium saucepan over medium heat, melt butter then whisk in brown sugar until glossy and no sugar granules remain visible, about 2 minutes. Remove from heat and let cool 5 minutes so you don't scramble the eggs.

Whisk in eggs one at a time until fully incorporated, then whisk in vanilla. The mixture should look thick and smooth, not greasy.

Switch to a rubber spatula and fold in flour, baking powder, and salt until just combined with no dry streaks. Do not overmix - a few small lumps are fine.

Spread two-thirds of the batter into the prepared pan. Drop half the cookie butter (about 6 tablespoons) in dollops over the surface. Use a knife to swirl gently. Spread remaining batter on top, then repeat with remaining cookie butter, creating a marbled pattern.

Scatter crushed Biscoff cookies over the top. Bake 26-30 minutes until the edges are deep golden and a toothpick inserted in the center comes out with a few moist crumbs. The center should look set but slightly underdone - it firms as it cools.

Cool completely in the pan on a wire rack, at least 1 hour. Lift out using parchment overhang, cut into 16 squares, and sprinkle with flaky salt if desired.