Preheat the oven to 450°F. Place the sliced stick of butter in an 8×8-inch baking dish. Bake 5–7 minutes, until melted and bubbling. Remove from the oven.
In a large bowl, whisk flour, baking powder, kosher salt, and sugar.
Add buttermilk and the 2 tablespoons slightly cooled melted butter. Stir just until combined. The batter will be thick.
Pour the batter directly over the melted butter. Do not stir.
Gently spread to the edges. Cut into 9 squares.
Bake 22–30 minutes, until golden and cooked through, with a lined sheet pan on the rack below to catch overflow. If the tops brown too fast, tent loosely with foil and bake about 5 minutes more.
Optional: brush the hot tops with melted butter and sprinkle lightly with flaky salt.
Rest 5 minutes before cutting.