Preheat oven to 350 degrees. Cream butter with lemon zest and sugar. Add the egg and vanilla and until combined. Meanwhile toss the blueberries with 1/4 cup of flour, baking powder and salt.

Add the flour mixture to the batter a little at a time, alternating with the buttermilk. Fold in the berries. Grease a baking tin and spread batter into the tin. Sprinkle over with sugar.