Bake sweet potato at 350° for 1 hour or until tender; cool slightly. Peel and mash; cool.
Combine flour and sugar in a medium bowl. Cut shortening and butter into flour mixture with a pastry blender until crumbly; add mashed sweet potato and milk, stirring just until dry ingredients are moistened.
Turn dough out onto a lightly floured surface; knead 3 to 4 times.
Roll dough to 1/2-inch thickness; cut with a 2-inch round cutter. Place on a lightly greased baking sheet.
Bake at 400° for 15 minutes or until golden brown.