Let the butter, buttermilk, eggs, blueberries, and lemon to come to room temperature. Takes about one hour.
Preheat the oven to 375.
Line 12 muffin tins with paper liners and spray with cooking spray.
In a medium sized bowl, beat the butter, sugar, and vanilla bean scrapings until you get a uniform, creamy mixture.
Add the eggs one at a time. Beat well and scrape the sides and bottom of the bowl after each addition.
Combine the flour, baking powder, and salt.
Add 1/2 of the dry ingredients to the creamed butter, sugar, and vanilla mixture and beat to just combine. Add 1/4 cup of the buttermilk and mix briefly. Scrape the sides and bottom of the bowl after each addition. Repeat till everything is just combined.
Mash 1/2 cup of the blueberries. Add the mashed and whole blueberries all at once. Stir to distribute the blueberries evenly throughout the batter. The batter will be thick and sticky.
Fill 12 muffin tins to the top. It may be easier to use your hands than a spoon. If you are using the crumb topping sprinkle it on top of the muffin batter.
Bake at 375 for 30 minutes, until a toothpick inserted into a middle muffin comes out clean.
Let cool on wire racks.