Preheat oven to 350ºF. In a mixing bowl, beat together the butter, lemon zest, and sugar (withholding 1 tablespoon) until the mixture becomes light and airy. Incorporate the egg and vanilla extract, blending until evenly mixed. In a separate small bowl, combine ¼ cup of the flour with the blueberries and toss to coat. Whisk the remaining flour with the baking powder and salt. Gently stir half of the flour mixture into the batter until partially integrated, then add all the buttermilk and mix to combine. Add the rest of the flour mixture, stirring just until no dry streaks remain. Carefully fold in the floured blueberries, excluding any excess flour from the bowl. Grease an 8- or 9-inch square baking dish with butter or non-stick spray; optionally, line with parchment for ease of removal. Transfer the batter into the pan and smooth the top. Sprinkle the reserved tablespoon of sugar evenly over the surface. Bake for 35–45 minutes, depending on pan size—closer to 35 minutes for a 9-inch pan and 40–50 minutes for an 8-inch one. Test for doneness using a toothpick. If needed, extend baking by several minutes. Allow to cool for a minimum of 15 minutes before slicing.