Preheat the oven to 325F/163C convection (300F/149C conventional) and grease and line an 8x8” pan with baking spray and parchment paper (leaving an overhang to easily remove them).
Place the butter in a medium saucepan over medium heat and stir intermittently with a rubber spatula or wooden spoon. When it begins to foam and bubble stir continuously, scraping the bottom of the saucepan until the butter has browned and smells nutty.
Pour the butter into a large bowl with the cookie butter and the light brown sugar and whisk aggressively for 1-2 minutes.
Add the eggs and vanilla and whisk aggressively for 2-3 more minutes until the mixture is completely smooth and pale.
Fold in the flour, baking powder, salt and ground Biscoff cookies, just until the last streak of flour is combined.
Spread half the batter into the prepared pan and place globs of cookie butter on top.
Spread the rest of the batter on top and then press the cookies on top of the batter.
Bake for 35-40 minutes, it’ll be golden brown and puffed in the center. When you press on it, it should feel soft but not liquid-y.
Remove from the oven and allow to cool for 30-60 minutes in the pan before slicing.
