Add all ingredients to the blender and blend to combine. Pour into the Ninja Creami container.
Add the lid, then freeze for at least 18 hours on a level surface in your freezer.
Remove from the freezer and run hot/warm water along the outer edge of the Creami container for 30 seconds to thaw the very outer layer.
Process on the Lite Ice Cream setting. If needed, add a splash of milk and respin.
Notes
Blending before adding to the Ninja Creami pint helps the cookie butter to get incorporated properly. If you freeze before blending, the cookie butter may get too hard when freezing and could damage the blades.
Thawing the outer edge (either by leaving out for 15 minutes or running warm/hot water along the edge for a minute) before processing will allow the outer layer to get mixed properly into the ice cream, otherwise the outer edge may stay frozen.
You can always add in broken speculoos cookies as a mix-in, crumble some on top, or serve with some on the side!