Preheat your oven to 400°F. Butter and flour an 8-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder + soda, salt, and cinnamon. Set aside. In the bowl of a stand mixer, beat together the butter and the ⅔ cup sugar until light and fluffy, about 2 to 3 minutes.
At this point, add in the vanilla and lemon zest, then the egg. Mix until all is well combined.
With the mixer on low, add the flour in three installments, alternating with the buttermilk in 2 installments. Mix until just combined. Transfer the batter to the prepared cake pan.
Drop the blueberries on top of the batter in the pan, it doesn't matter how you arrange them, they'll sink into the cake as it bakes. Mix together the cinnamon sugar topping and sprinkle evenly over the entire batter.
Bake the cake for 20 to 25 minutes, until the top is golden and a toothpick or cake tester comes out clean. Cool the cake in its pan for 10 minutes, then invert onto a plate to serve. Best eaten warm!