Preheat the oven to 350 degrees. Butter and flour a 9-inch springform pan and place it on a sheet pan. Place the plums, tapioca, creme de cassis, and 3/4 cup of the sugar in a mixing bowl and stir to combine. Allow to sit for 15 minutes. In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and remaining 1 cup of sugar on medium speed until light and fluffy. In a small bowl, combine the flour, cinnamon, salt, and baking powder. With the mixer on low, gradually add the dry ingredients to the butter mixture until it forms small, dry crumbs. Add 1 tablespoon of cold water and continue to beat for about 30 seconds, until the mixture forms large, moist crumbs. Set aside 3/4 cup of the crumb mixture and pour the rest into the springform pan. With floured hands, lightly pat the dough evenly in the bottom of the pan and 1 inch up the sides. Arrange the plums in concentric circles on the crust. Sprinkle the remaining crumb mixture evenly on top. Bake for 1 hour, until the fruit is bubbling and the crust is golden. Cool for 15 minutes, then remove the sides of the pan and serve warm or at room temperature.
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