Preheat oven to 350°F. Grease and flour a 9×13-inch baking pan.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
Add the softened butter, eggs, and milk. Mix on low speed until combined, then beat on medium speed for 2 minutes, until smooth. The batter will be thick.
Fold in 1 cup of the blueberries by hand.
Spread the batter evenly into the prepared baking pan.
Scatter the remaining 1 cup of blueberries over the batter. In a small bowl, stir together the sugar and cinnamon, then sprinkle evenly over the blueberries.
Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean and the edges are lightly golden brown.
Cool slightly before serving warm or at room temperature.
