How to Make It
Combine flour, baking powder, salt, and baking soda in a large bowl. Cut in butter until crumbly. Stir in buttermilk.
On a work surface, fold (don’t knead) the dough until it just comes together; shape into a rectangle. Cut into 8 squares.
Brush squares with beaten egg. Bake in a 375° F oven until golden brown, 25 to 30 min.