Consistency-wise, nut butter or other creamy and thick dessert spreads (like Nutella) make a good substitute for cookie butter. But note that the flavor of the shake will be very different depending on what spread is used. Cashew butter and almond butter have the most neutral taste, compared to peanut butter. Add a pinch of pumpkin pie spice to give the milkshake a bit of that speculoos cookie flavor.