Preheat the oven to 350 degrees F and put in a baking sheet to heat up.
Put the rhubarb into a saucepan with the 1/3 cup of sugar. Put a lid on the pan and simmer gently for about 10 minutes, until the rhubarb is soft. Put aside and leave to cool completely.
Get yourself 6 ramekin dishes and rub their insides with the butter. Put the gingersnap cookies into a sandwich bag, tie a knot in the top and smash the cookies with a rolling pin or the bottom of a pan to make quite fine crumbs. Dust the insides of the buttered ramekins with the smashed cookies, then shake out any excess crumbs and keep them for later. (You can put the dishes into the refrigerator at this point until you're ready to put your souffles together.)
Blob a tablespoon of the cooled stewed rhubarb into each ramekin dish. Mix the rest of the rhubarb with the custard, the egg yolk and the flour. In a large, clean bowl, using an electric mixer, beat the egg whites with a pinch of salt until you have soft peaks. Add the remaining 2 tablespoons of sugar and whisk on a high speed until the whites are very stiff - this should take about 3 minutes.
Working gently, fold 2 spoonfuls of the stiff egg whites into the rhubarb mixture. Tip this into the bowl containing the remaining egg whites and fold together very carefully. Divide the mixture between the ramekins and level the tops. Wipe the rims of the dishes clean.
Remove the hot baking sheet from the oven and place the ramekins on it. Put back into the preheated oven and bake for 18 to 20 minutes, or until the souffles are a lovely golden color and have risen nicely. Serve immediately, sprinkled with your leftover gingersnap crumbs.
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Recipe Excerpted from JAMIE AT HOME by Jamie Oliver. Copyright (c) 2008. Published in the U.S. by Hyperion. All Rights Reserved. www.jamieoliver.com