First make the streusel. In a medium bowl mix the flour, brown sugar, and salt. Then, using your fingers, rub the butter into the flour mixture until there are large clumps (about 1/8 inch large). Place the streusel mixture into the freezer.
For the cake, sift together the flour, baking powder, baking soda, and salt. Set the dry ingredients aside.
In the bowl of a standing mixer fitted with a paddle attachment, cream the butter and sugar until light and fluffy, about 4 minutes on medium.
Add the eggs and vanilla. Mix until just combined.
Add the dry ingredients and the sour cream into the batter in 3 additions, starting and ending with the dry ingredients.
Cover the batter and refrigerate for about 30 minutes.
Preheat the oven to 325 degrees F.
While the batter rests, line the - inch, springform baking pans with parchment rounds. Spray the pans with baking spray.
Place the prepared pans on a sheet pan.
Mix the blueberries with the brown sugar.
Once the batter is chilled, half fill each cake pan.
Divide the blueberry mixture between the cake pans.
Cover the blueberry mixture with the balance of the batter.
Spinkle each cake with streusel mixture.
Bake for 35 to 40 minutes.
Allow the cakes to cool completely. Then remove the cakes from the pan and sprinkle with powdered sugar.