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.

Preheat oven to 350F/180C. Butter a 9x13-inch baking pan.

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.

Add the plums to the streusel mixture and toss. Sprinkle the topping evenly over the batter.

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.

Cake can be kept in the fridge, covered, for up to 4 days.