Line a 9x13-inch baking dish with parchment paper. Preheat oven to 350 degrees.
Whisk together flour, brown sugar, baking powder, and salt in a large bowl.
Add egg, melted butter, milk, and vanilla extract and stir with a wooden spoon or rubber spatula until combined.
Press evenly into the bottom of the prepared pan.
To make the filling, place cream cheese in a large bowl. Use a hand-held mixer to beat it until smooth and creamy.
Add biscoff and beat until mixed.
Beat in eggs, then melted butter and vanilla.
Add powdered sugar in 3 additions, beating until incorporated.
Pour filling over crust. Bake for 40 to 45 minutes. You still want it somewhat gooey, so be careful not to overbake. Let cool before slicing.