Pipe roughly ⅓ of your Biscoff buttercream on top of the bottom layer and sprinkle about ½ of your cake crumb. Use your hands to gently press the crumb into the frosting so it sticks to it. Add another coat of frosting on top to hold the crumb in place.
Use a spoon to swipe small dollops of the melted spread around your cake. Then place a cake scraper against the side of the cake and gently pull the cake around to smooth out the frosting and create a marble design.
Fill in any gaps in the frosting with your Biscoff buttercream, but refrain from marbling and smoothing the cake more than 3 times. The spread will start to harden, making it messier and more difficult to smooth the cake’s sides.