Recipe discovered among the treasures of sneff on everything2.com, a suitable IceCream to pair with VanillaFrozenCustard or to eat on its own. Herein are notes from the production experiment:

A quarter of a small watermelon should provide just over two cups of watermelon juice. If you buy a seedless, you can whiz it in a blender instead of pressing it through a sieve: Slice down a wedge to the rind in several spots, then cut along the rind to drop chunks into the blender pitcher. When the loosely-stacked chunks reach the 1-liter mark, you have just over 2 cups' worth of juice. High speed is not important, but pulsing is—in both cases because the watermelon immediately near the blade will disintegrate pretty much instantly, without exerting any force on its neighbors.

Catch the sorbet very early in its setting or the egg whites will not integrate so well. Check it after 20, 30, and 40 minutes if you need to. This stuff will freeze solid in an hour or so, while the vanilla custard is still at the soft-serve consistency. Take the sorbet out, put it in the freezer, and add the ice and brine to that used by the custard.