Preheat the oven to 350˚F.
In a large bowl, cream the butter and sugar together.
Mix in the eggs and vanilla, then stir in the buttermilk.
Add one cup of flour, along with the salt and baking powder. Mix.
Then add the remaining cup of flour and mix until just moistened.
Fold the blueberries.
To make the streusel topping, mix the softened butter with brown sugar and flour until combined and crumbly.
Grease a 2 quart baking dish and spread the batter evenly.
Top with the streusel topping.
Bake for 35-45 minutes, or until a toothpick inserted in the middle comes out clean.
Once done, let cool for 15-20 minutes, then cut into 12 pieces.
