Grease or line a 9x13 inch baking pan with parchment.
Make the butter cake layer
In a large bowl, whisk together the flour, sugar, baking powder and salt. Beat in the egg, melted butter, milk and vanilla and mix until just combined. Using a spatula or the back of a spoon, firmly press the mixture into the prepared baking pan.
Make the gooey Biscoff layer
In the bowl of a stand mixer or in a large bowl if using a handheld mixer, beat the cream cheese on medium to medium-high speed until it is smooth and creamy. Add the eggs, vanilla and Biscoff and continue to beat until the ingredients are well incorporated.
Turn the mixer to low and slowly add the powdered sugar a little bit at a time until all the powdered sugar has been incorporated. Do not over mix.
Pour the gooey batter onto the already-pressed cake layer and spread evenly with a spatula.
Bake the bars for 35 – 40 minutes or until the tops appear shiny and the edges are a light golden color. The center may appear to be slightly underbaked but it will continue baking and set.
Turn off the oven and remove the bars. Let the bars cool and come to room temperature before slicing.
The bars can be stored at room temperature in an airtight container for about one week.
Source: Butter Baking; originally adapted from Bake Your Day