IN A MEDIUM BOWL, whisk together the flour, powdered sugar, baking powder, and salt. Set aside.
In another medium bowl, whisk together the buttermilk, egg, melted butter, and vanilla. Slowly whisk in the flour mixture, mixing until smooth. The batter will be thick.
Heat a large cast-iron skillet or griddle over low heat for about 5 minutes. Add a tablespoon of butter to the pan and swirl it around to coat. (The butter should sizzle slightly when it hits the pan.)
Drop 1/4-cup scoops of batter into the pan, leaving an inch or two between the cakes. Let cook undisturbed until the edges start to bubble, about 2 minutes. Flip each cake and cook for another 2 minutes, until golden brown around the edges and cooked through the center. Continue cooking the remaining batter in batches.
Serve immediately with softened butter and warm maple syrup.