Preheat oven to 375°F (195°C). Grease an 8-inch oven-safe skillet with butter and dust with flour. Set aside.
Whisk together flour, baking powder, baking soda and salt. Set aside.
With a stand mixture or electric mixer on medium speed, beat butter and 3/4 cup sugar until pale and fluffy, 3 to 5 minutes. Beat in the egg. Slowly add in the flour mixture in 3 batches, alternating with the buttermilk. Then beat until combined.
Pour the batter into the prepared skillet and smooth the top. Sprinkle on the fresh blueberries. Then sprinkle the remaining 2 tablespoons sugar over top.
Bake the cake until golden brown and a cake tester into center comes out clean, around 35 to 40 minutes.
Transfer to a wire rack to cool slightly before serving. Enjoy!