Preheat oven to 425 degrees. Line a muffin tin with paper liners.
Add the butter, brown sugar, and sugar to a mixing bowl and beat on medium speed until light and fluffy.
Beat in the eggs until well incorporated.
Stir in the sour cream, buttermilk, and vanilla until combined.
Add the flour, baking soda, baking powder, and salt to the bowl and stir by hand until the flour has just disappeared into the batter. Do not overmix or your muffins will be tough.
Gently fold the blueberries into the batter with a rubber spatula.
Fill the muffin tins 2/3 full (a large cookie scoop works perfectly for this) and top with the streusel.
Sprinkle the streusel over the muffin tops generously. For large chunks of streusel, compact the topping with your hand before sprinkling over the muffins. For a finer streusel topping, break the streusel up before sprinkling.
Bake at 425 degrees for 5 minutes and then reduce the heat to 350 and finish baking for 20 minutes or until the tops spring back when lightly pressed and a toothpick insered in the center comes out clean.
Cool for 10 minutes in the muffin tin before removing to the counter to cool completely.