Prepare the Filling: Preheat oven to 375 degrees Fahrenheit. Remove the pits from the plums and slice them into ½-inch pieces.
2 pounds plums
In a mixing bowl, stir together the brown sugar, granulated sugar, flour, cardamom, cinnamon, and ginger, then toss the plums in this mixture. Stir in the lemon juice then set aside.
¼ cup brown sugar, ¼ cup granulated sugar, ¼ cup all-purpose flour, ½ teaspoon ground cardamom, ½ teaspoon ground cinnamon, ¼ teaspoon ground ginger, ¼ cup lemon juice
Make the Topping: In another mixing bowl, stir together the flour, brown sugar, baking powder, cinnamon, sea salt, vanilla extract, and egg. The mixture will look like a crumbly dough.
1 cup all-purpose flour, ½ cup brown sugar, 1 teaspoon baking powder, ¾ teaspoon ground cinnamon, ½ teaspoon fine sea salt, 1 teaspoon vanilla extract, 1 egg
Assemble and Bake: Pour the plum filling mixture into a baking dish then sprinkle the topping mixture over it evenly.
Spoon the melted butter evenly over top and transfer to the oven to bake for about 30 minutes or until the topping is golden brown and the plums are tender and bubbly. Serve warm.
½ cup unsalted butter