Preheat oven to 400 degrees; line muffin cups with paper baking cups. In a medium bowl, stir together the flour, sugar, cocoa, baking powder, baking soda and salt. Stir in the yogurt, milk and vanilla extract just until combined. Do not beat. Fill the muffin cups two third full with batter.
Bake for 15 to 20 minutes, or until a toothpick inserted in the center comes out clean. Cool slightly in the pans on wire racks. Remove from the pans. Sprinkle powdered sugar over the tops of the muffins, if desired. Serve warm. Cover and store at room temperature, or freeze in an airtight container for longer storage.