Whisk the flour, herb mixture, baking powder, baking soda, salt and 1/4 teaspoon pepper. Cut in the butter with a pastry blender until sandy. Stir in the buttermilk.
Turn out onto a floured piece of parchment paper. Pat into a 3/4-inch-thick rectangle.
Cut the dough into rough 2-inch squares with a large knife. Cover with plastic wrap.