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.
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.