Preheat oven to 375 degrees. Whisk flour, baking powder, salt, and sugar in a large bowl. Cut in butter with a pastry blender (or rub in with your fingers) until pea-size crumbs form. Cut in yolk. Stir in buttermilk until just combined.
Turn dough out onto a lightly floured surface, and knead twice. Pat dough to a 1-inch thickness. Using a 2 1/4-inch round cutter, cut out biscuits, and transfer to a nonstick baking sheet. Repat dough scraps, and repeat once.
Lightly brush biscuit tops with buttermilk. Bake until tops are golden brown, 20 to 22 minutes. Serve hot with butter and pepper jelly on the side. (Unbaked biscuits can be frozen up to 1 month. Brush with buttermilk, and bake 32 to 35 minutes.)
Mixing a mashed hard-cooked egg yolk into the dough creates a tender, delicate texture in the resulting biscuits.