"Draining the rhubarb will keep the cake from becoming soggy...."
INGREDIENTS
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4 tablespoons unsalted butter, softened, plus more for pan
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1/4 cup all-purpose flour, plus more for pan
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5 ounces rhubarb, cut into 1/4-inch-thick slices (scant 1 1/2 cups)
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1 cup plus 1 tablespoon sugar
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1/2 teaspoon baking powder
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Pinch of coarse salt
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2 large eggs
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1 vanilla bean
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1 large handful blackberries (about 1/3 cup)