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1
Preheat the oven to 350 degrees.
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2
Butter and flour a 9-inch springform pan and place it on a sheet pan.
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3
Place the plums, tapioca, creme de cassis, and 3/4 cup of the sugar in a mixing bowl and stir to combine.
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4
Allow to sit for 15 minutes.
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5
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.
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6
In a small bowl, combine the flour, cinnamon, salt, and baking powder.
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7
With the mixer on low, gradually add the dry ingredients to the butter mixture until it forms small, dry crumbs.
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8
Add 1 tablespoon of cold water and continue to beat for about 30 seconds, until the mixture forms large, moist crumbs.
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9
Set aside 3/4 cup of the crumb mixture and pour the rest into the springform pan.
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10
With floured hands, lightly pat the dough evenly in the bottom of the pan and 1 inch up the sides.
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11
Arrange the plums in concentric circles on the crust.
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12
Sprinkle the remaining crumb mixture evenly on top.
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13
Bake for 1 hour, until the fruit is bubbling and the crust is golden.
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14
Cool for 15 minutes, then remove the sides of the pan and serve warm or at room temperature.
