Black paint marker 2-4 mm
Does your window fog up, are there grabby fingers/curious noses nearby or would you like to have the drawing stay on longer? Then it's better to draw the design with a paint marker. This is water-, smudge-, light- and heatproof after it dries. Also nice: it's odor-free pigment ink. The marker is not food-safe.
These markers are suitable to draw on glass, mirrors and other smooth, non-porous surfaces*. And when you're done with the drawing you can easily remove it with turpentine diluted with water 1 to 2 (i.e. 1 cup turpentine + 2 cups water). Not suitable for children. NOTE! THIS IS NOT A CHALK MARKER
brand Edding | type 750
stroke 2-4 mm, bullet nib
Shake the marker (cap on!), then pump on a piece of scrap paper until you see the ink flow into the nib. Repeat this every now and again when you're drawing to get smooth lines.
If you buy a marker with a window drawing design, you will receive the marker in the mail and the printable via email. The window drawing designs are not physical products, but something you print it at home.
* Not ideal on chalkboard(paint) because that's too porous and you won't be able to completely remove it.