Preheat oven to 350-degrees. Prepare a 9x13 pan with aluminum foil or parchment and apply nonstick spray.
In a mixing bowl, cream together the butter, Biscoff spread (or nut butter), and sugars. Add the eggs and blend in, followed by all of the dry ingredients. Gently fold in marshmallows and chocolate chips. It'll be thick.
Spread the dough into the pan and even out. Bake for 20 to 25 minutes, or until lightly browned on the edges. The center will jiggle slightly from the marshmallows but it will set after it cools.
Cool completely and cut into squares. Store in a tightly sealed container.