Preheat oven to 325 degrees. Spray a 10-inch spring form pan lightly with cooking spray.
Set aside a ½ cup of blueberries to put on top of the cake. Toss the remaining 2 cups of berries with 1 tablespoon of flour and set aside.
In a small bowl, sift together flour and baking powder. Whisk in salt and set aside.
In a stand mixer, cream together butter and sugar. Add the egg and beat until pale and fluffy. Beat in vanilla and half of the lemon zest. Set the other half of lemon zest aside.
Alternate adding in the flour mixture and buttermilk to the stand mixer, about a third at a time. Scrape down the sides between each addition. Beat until well incorporated. Batter will be fairly thick.
Using a rubber scraper, gently fold in the 2 cups of blueberries until fairly evenly distributed throughout the batter.
Spoon batter into the spring form pan. Use a spatula to smooth the batter to the edges of the pan. Press the remaining half cup of blueberries into the top of the batter.
In a small bowl, use fingers to mix together the coarse sugar and remaining lemon zest. Sprinkle over top of the cake.
Place cake on the middle rack of the oven. Bake for about 50 minutes, or until a toothpick inserted comes out clean.
Let cool about 15-20 minutes before removing the sides of the spring form pan.
Serve warm. Can be served with fresh whipped cream and powdered sugar for a dessert feel.
Cake will keep in the refrigerator for about 5 days when loosely covered.