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The crumb topping is a key feature, made from half a cup of melted butter, one cup of flour, a teaspoon of kosher salt, and half a cup of sugar. Whisked together, then stirred into the butter to form coarse crumbs, this topping adds texture and crunch. Extra butter cubes are scattered over the rhubarb base before baking, melting beneath the fruit and creating a glistening, irresistible finish.