In a large mixing bowl using an electric hand mixer, cream together the softened butter and sugar for two or three minutes. Add egg, vanilla, and lemon zest. Beat another minute or two, or until well combined. Add flour, a little at a time, alternating with buttermilk, beating just until each addition is incorporated. Using a wooden spoon, gently fold in the blueberries. Batter will be thick. Transfer batter to prepared baking dish.