I've posted about it before (here) - a technique using a stamping pad, which produces a great clean effect.
But because I was using white "ink" this time, I painted it onto the stamps:
What you'll need:
Cookies flooded with black icing - that have dried for at least 24 hrs
Rubber stamps - used solely for food, they must not have come in contact with non-edible ink
White gel colour- mixed 1:1 with clear alcohol
A firm, square-tipped paint brush
Royal icing - for additional details
(It's a good idea to have a few spare cookies to practice the technique on!)
Paint the white gel mix onto the surface of the stamp - make sure that all the details are covered, but that there aren't any blobs obscuring fine details.
Press the stamp firmly onto the flooded cookie - apply extra pressure in the centre which is where there is often a slight dip of icing.
Allow to dry.
Add royal icing details.
(If you want the speckled effect that I've created on these cookies, dip the paintbrush in the white paint and run your finger over the bristles about 20cm above the cookies.)