Combine 1/2 cup flour, coconut, 1/2 cup brown sugar, and cinnamon in a bowl. Toss in melted butter and set aside.
Preheat the oven to 325°F Lightly coat a 9-inch round springform pan with a little of the softened butter and set aside. Combine the remaining flour, baking powder, and salt in a bowl and set aside. Beat the egg yolks, remaining brown sugar, granulated sugar, and vanilla using an electric mixer set on high speed until the mixture is thick and pale yellow, about 2 minutes. Mix in the remaining butter and the sour cream. Reduce mixer speed to low, add flour mixture, and mix.
Pour batter into the prepared pan, sprinkle with topping, and bake on the middle rack of the oven until a toothpick inserted into the center of the cake comes out clean, 55-60 minutes. Cool in pan on a wire rack for 1 hour. Release the cake from the pan. Serve warm. Enjoy!