Preheat the oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper.

In a mixing bowl, whisk together ½ cup melted unsalted butter, ½ cup granulated sugar, and ½ cup light brown sugar until glossy.

Incorporate 2 eggs and 1 tsp vanilla extract and whisk until thick and creamy.

Sift in 1½ cups flour, 1 tsp baking powder, and ½ tsp salt, folding gently until just combined.

Fold in 1 cup chopped Biscoff cookies and ½ cup white chocolate chunks.

Spread half of the batter into the prepared baking dish, smoothing it out.

Microwave ½ cup Biscoff cookie butter until pourable and drizzle over the first layer.

Spread the remaining batter over the cookie butter layer and sprinkle with reserved cookie chunks.

Bake for 35-40 minutes until edges are golden and a toothpick comes out mostly clean.

Allow to cool completely in the pan before cutting into squares.