Preheat the oven. First, preheat your oven to 350ºF (175ºC) and grease a 9x9-inch baking dish to prevent the cake from sticking.
Toss blueberries. In a bowl, toss 2 cups fresh blueberries in 1/4 cup all-purpose flour. This helps prevent them from sinking to the bottom of the cake while baking. Set aside.
Cream the butter and sugar. In a large mixing bowl, cream together 1/2 cup unsalted butter (softened), 1 cup sugar, and 2 tbsp lemon zest. Beat for 3-5 minutes until the mixture is fluffy and light.
Add the egg and vanilla. Beat in 1 large egg and 1 1/2 tsp vanilla extract until thoroughly incorporated.
Combine dry ingredients. In a separate bowl, whisk together the remaining 1 3/4 cups all-purpose flour, 2 tsp baking powder, 1 tsp salt, and 1/2 tsp cinnamon (if using).
Mix dry and wet ingredients. Gradually add the dry mixture to the creamed mixture, alternating with 1/2 cup buttermilk, beginning and ending with the flour mixture. This method ensures a smooth, well-combined batter.
Fold in blueberries. Gently fold the floured blueberries into the batter using a spatula.
Transfer to dish. Pour the batter into the prepared baking dish; smooth the top with a spatula. Sprinkle the top with 1 tbsp sugar for a crunchy topping.
Bake. Bake the cake in the preheated oven for 40-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool. Allow the cake to cool on a wire rack for at least 15 minutes before slicing.