Preheat oven to 350°F (177°C). Line the bottom and sides of an 8-inch square baking pan with aluminum foil or parchment paper, leaving enough overhang on all sides. Set aside.

In a medium sized bowl, toss together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, stir the melted butter and brown sugar together until combined. Whisk in the egg and egg yolk, then add the vanilla. Stir in the Biscoff spread. Gently fold the dry ingredients into the wet ingredients. Be careful not to overmix, which will result in crumbly, hard blondies. The batter will be very thick. Fold in the white chocolate chips.

Spoon the batter into the prepared baking dish. Bake for 25-26 minutes. The blondies may appear very soft, but they will set up as they cool. Allow the blondies to cool completely- about 3 hours. Lift the foil or parchment out of the pan using the overhang on the sides and cut into bars.