Preheat oven to 350° F.
Using a stand mixer, hand mixer, or whisk, combine the butter, lemon zest, and sugar.
Add the egg and vanilla extract and mix until combined.
Toss the blueberries in enough flour to lightly coat them. This helps them distribute throughout the batter.
In a separate bowl, combine the flour, baking powder, and salt.
Add your flour mixture and buttermilk to your butter mixture and use a spatuala to fold together and combine.
Add your blueberries and carefully fold them in with a spatula to combine.
Grease an 8 x 8 baking dish.
Pour your batter into the dish and spread it evenly.
Sprinkle the top of your batter with sugar.
Bake for about 40-45 minutes. Use a toothpick to check if it's done.