Preheat the oven to 350F. Grease an 8x8-inch baking dish. Set aside.
In a small bowl, toss the blueberries with 1/4 cup flour. Set aside.
In a large bowl, beat the butter, lemon zest, and sugar together until well-combined, about 2-3 minutes on medium.
Mix in the egg and vanilla extract until well-combined.
Add the remaining flour, baking powder, and salt.
Stir in just until combined. Don't overmix.
Add the buttermilk.
Stir in just until combined. Don't overmix.
Fold in the blueberries just until incorporated.
Pour the batter into the prepared pan.
Sprinkle with remaining sugar.
Bake for 35-50 minutes or until a toothpick inserted in the center comes out clean.
Cool 15-20 minutes before slicing and serving.
Notes
Blueberries: Be sure to wash them and then pick them over so that you don't have any smooshy or rotten blueberries in your cake.
Flour: Be sure to properly measure the flour. To properly measure the flour, sift or stir it to break it up. Lightly spoon into the measuring cup and then level off.
Butter: I use salted butter. Feel free to use unsalted if you prefer.
Buttermilk: If you don't have buttermilk, you can make a substitute with this recipe.
Room temperature: Be sure to have you butter, egg, and buttermilk at room temperature. This will help the ingredients to combine fully.