Preheat your oven to 350°F (175°C).

Grease a 9x9 inch baking pan.

In a medium bowl, cream together softened butter, sugar, and lemon zest until light and fluffy (about 3-5 minutes with a mixer).

Add the egg and vanilla extract, beating until combined.

In another bowl, whisk together flour, baking powder, salt, and cinnamon (if using).

Toss blueberries with a bit of flour to prevent sinking.

Gradually add dry ingredients to the wet ingredients, alternating with buttermilk, starting and ending with dry ingredients. Mix until just combined.

Gently fold in the blueberries.

Spread the batter evenly into the prepared pan and sprinkle with extra sugar.

Bake for 35-45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Let the cake cool in the pan for at least 15 minutes before slicing and serving.