Combine yeast and 1/2 cup warm water in a 4-cup liquid measuring cup, and let mixture stand 5 minutes. Stir in buttermilk.
Combine flour and next 4 ingredients in a large bowl; cut in shortening with a pastry blender or fork until mixture resembles coarse meal. Add buttermilk mixture, stirring with a fork just until dry ingredients are moistened.
Turn dough out onto a well-floured surface, and knead 4 to 5 times.
Roll dough to 1/2-inch thickness; cut with a 2-inch round cutter, and place on lightly greased baking sheets. Cover and let rise in a warm place (85°), free from drafts, 1 hour.
Bake at 425° for 10 to 12 minutes or until golden. Split each biscuit, and spread evenly with Mustard Butter. Stuff biscuits with ham.