Preheat oven to 450F. Line a large baking sheet with parchment paper.

In bowl, combine flour, cream of tartar, baking soda and salt. Cut cold butter or shortening into dry ingredients with a pastry blender. (Mixture will resemble coarse crumbs, with no large chunks of butter.) If butter gets very soft at this point, refrigerate mixture for 20 minutes.

Add 1.75 cups buttermilk, stirring just to moisten all ingredients. Dough should be soft and moist; add remaining 1/4 cup as needed.

Turn dough out onto lightly floured work surface and knead gently about 10 times, or just untll dough holds together. Roll or pat dough into a 14-by-10-inch rectangle. With short side nearest you, spread top two-thirds of dough with 3 tablespoons soft butter, leaving bottom third, closest to you, unbuttered. Fold dough into thirds by pulling bottom third up over center and then pulling top third over middle.

Turn dough so short side faces you. Pat into a 9-by-12-inch rectangle.  In same manner, spread again with 3 tablespoons soft butter and fold letter style. Turn once more in the same manner. Pat into a 9-by-12-inch rectangle; spread with remaining 3 tablespoons soft butter and fold up. Work quickly and gently so as not to overwork dough.

Pat dough into rectangle 3/4-inch thick on floured surface. Cut into squares with a sharp chef’s knife. Place on pan, 1 inch apart. Lightly brush tops with melted butter. Bake in center of hot oven about 20 minutes (about 12 minutes in convection), until golden brown and firm. Serve hot, warm, or room temperature.