Grease a nine inch square cake pan, and then line it with parchment paper so that some paper hangs over two of the sides. That way you'll be able to lift it out of the pan.
In a large bowl, cream together the butter and granulated sugar. Add the eggs, buttermilk, and vanilla and stir until well combined.
Using a sifter, add the flour, baking powder, and salt to the bowl. Stir until just combined. Stir in 1½ cups of the blueberries.
Pour the batter into the pan and bake on the middle rack for twenty -five minutes. Then, sprinkle the cinnamon and brown sugar over the top of the cake. Sprinkle on the rest of the berries and bake about ten minutes more, or until a toothpick inserted into the middle of the cake comes out clean.
Cool the cake in the pan on a wire rack for ten minutes. Then, remove the cake from the pan and let it sit until cool.