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Prepare the streusel: use a fork to mix the flour with the brown sugar, ginger, and salt. Drizzle in the melted butter and stir until it resembles a clumpy dough. Add the coconut and crystallized ginger, and use your fingers to break the mixture into clumps. Refrigerate.
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Preheat oven to 350F/180C. Butter a 9x13-inch baking pan.
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Make the cake: in a medium bowl, whisk the flour with the baking powder, baking soda, and salt. In a large bowl and using an electric mixer, beat the 1 cup butter with sugar at medium speed until fluffy, 2 minutes. Beat in the eggs, one at a time, and then the sour cream and vanilla. Beat the flour mixture into the batter in 3 additions until just incorporated. Scrape the batter into the pan and spread it to the edges.
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Add the plums to the streusel mixture and toss. Sprinkle the topping evenly over the batter.
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Bake the cake for 45-60 minutes until the crumb topping is browned and a cake tester inserted in the center of the cake come out clean. Transfer to a wire rack and let cool completely.
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Cake can be kept in the fridge, covered, for up to 4 days.