Combine yeast and 1/4 cup warm water in a glass measuring cup; let stand 5 minutes.
Stir together sweet potatoes and butter, blending well. Stir in sugar and salt; add yeast mixture, stirring until smooth.
Combine flour and baking powder; gradually stir into potato mixture, until well blended. Lightly knead until dough holds together.
Shape dough into a ball; place in a buttered mixing bowl. Brush top with melted butter. Cover and let rise in a warm place (85°), free from drafts, 2 1/2 hours or until doubled in bulk.
Punch down dough, and turn out onto a floured surface. Roll dough to a 1/2-inch thickness; cut with a 2-inch round cutter, and place on greased baking sheets. Cover and let rise in a warm place, free from drafts, 2 hours or until doubled in bulk.
Bake at 400° for 12 minutes or until golden.