This is one of my absolute favorite summer recipes. With most blueberry coffee cakes that have a crumb topping, the crumbs never get 100% crunchy. I found a solution for getting juicy, sweet blueberries on top, moist cake in the middle, and a bottom layer of surprisingly crunchy crumbs.
The fruit: Preheat oven to 350 F. Grease a 9-inch round cake pan with butter. Line with parchment paper and grease the top of the paper. In a small saucepan, melt 3 tablespoons butter over medium-low heat. When melted, add lemon juice and brown sugar. Stir until sugar has dissolved. Pour the liquid into the bottom of the prepared cake pan. Pour the blueberries into the bottom of the pan. This can be done in advance.
Crumb topping: Mix together flour, sugars, and spices in a medium bowl. Pour in the melted, cooled butter and mix until large crumbs form.
Cake: In the bowl of a standing mixer fitted with the paddle attachment, combine flour, sugar, baking soda, baking powder, and salt. In a mixing bowl, whisk together the eggs, sour cream, and vanilla. Attach the bowl with the dry ingredients to the standing mixer and mix for a few seconds. Add the softened butter and a few tablespoons of the egg mixture, and beat for 10 seconds to combine. Beat in the remaining egg mixture, continuing to beat until very smooth.
Pour the cake batter on top of the fruit and smooth the top. Use your fingers to create large crumbs from the topping and scatter evenly on top of the cake batter. Bake 55–65 minutes, until cake is set and the center springs back when very gently pressed with a finger. A cake tester will come out with crumbs attached, but it shouldn't be wet.
Remove cake from oven and transfer to a wire rack to cool completely, at least 2 hours.Use a flat-edged spatula or knife to loosen the sides of the cake from the pan. Flip cake onto the rack. Serve with the fruit either on the bottom or on top, whatever your preference.