Heat oven to 400 degrees. Line muffin cups (2 1/2 inch in diameter with paper bake cups. In medium bowl, stir together flour, granulated sugar, cocoa, baking powder, baking soda and salt; stir in yogurt, milk and vanilla just until combined. Do not beat. Fill muffin cups 2/3 full with batter.
Bake 15-20 minutes or until wooden pick inserted in center comes out clean. Cool slightly in pan on wire rack. Remove from pans. Sprinkle powdered sugar over tops of muffins, if desired. Serve warm. Store, covered, at room temperature or freeze in airtight container for longer storage.
NUTRITIONAL INFORMATION PER MUFFIN: 100 calories; 2 grams protein; 22 grams carbohydrates; 1 gram fat; 0 milligrams cholesterol; 200 milligrams sodium; 45 milligrams calcium.