In a standing mixer combine the butter, Biscoff butter, brown sugar and granulated sugar. Mix until combined, then turn the speed of the mixer up to high and whip for 1-2 minutes.
Add the vanilla and eggs, stir until combined.
Add the flour, salt & cream of tartar. Mix until the dough forms.
Pour the crushed Biscoff cookies into the mixture and fold in
Spread the batter out onto a sprayed 9x13 pan
Bake for 30 minutes.
Remove and allow to cool for 10 minutes before slicing.