1. Heat oven to 400°F. Line muffin cups (2-1/2 inches in diameter) with paper bake cups.
2. Stir together flour, granulated sugar, cocoa, baking powder, baking soda and salt in medium bowl; stir in yogurt, milk and vanilla just until combined. Do not beat. Fill muffin cups 2/3 full with batter.
3. Bake 15 to 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. Makes about 12 muffins.