Preheat your oven to 350°F (175°C). With cooking spray, spray a 9-inch round springform cake pan and line the bottom with parchment paper, then spray again for extra non-stick insurance.
Begin by browning the butter. In a small saucepan over medium heat, melt the unsalted butter. Let it cook, swirling occasionally, until it turns a golden brown color and gives off a nutty aroma. This should take about 5-7 minutes. Be careful not to burn it. Once browned, remove from heat and allow it to cool slightly.
In a large mixing bowl, whisk together the brown sugar and the browned butter until well combined.
Add the eggs one at a time, mixing well after each addition (about 30 seconds per egg). Then stir in the vanilla extract and sour cream until smooth.
In a separate bowl, sift together the all-purpose flour, baking powder, and baking soda. Using a rubber spatula, gradually fold the dry ingredients into the wet ingredients, mixing until just combined. Ensure the batter is smooth and well incorporated.
Gently fold in the fresh blueberries until evenly distributed throughout the batter.
Pour the batter into the prepared cake pan, spreading it out evenly. With the back of a spoon, gently spread the blueberry jam on top, ensuring an even layer across the top of the cake batter. Set aside while you make the streusel.
For the Streusel:
In another bowl, prepare the streusel topping. Combine the light brown sugar, all-purpose flour, ground cinnamon, and salt. Using your fingers or a pastry cutter, cut in the cold cubed butter until the mixture resembles coarse crumbs. This takes some time, so be patient. You want a shaggy look.
Sprinkle this streusel topping evenly over the jam spread in the cake pan. Optionally, top with a few extra blueberries for decoration.
Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and allow it to cool in the pan until it has come to room temperature. This helps the cake set properly.
Once the cake has cooled, dust the top generously with powdered sugar. Trust me, this part is what makes it taste SO good.
Serve slices of the brown butter blueberry sour cream coffee cake and enjoy!