Preheat oven to 350°F. Line twenty-one 2½-inch muffin cups with paper bake cups or coat with nonstick cooking spray; set aside. In a large bowl, combine flour, cocoa powder, espresso powder, baking powder, baking soda and salt.
In another large bowl, whisk together buttermilk, granulated sugar, oil, eggs and vanilla. Add buttermilk mixture to flour mixture. Beat mixture with a wire whisk just until combined.
Fill muffin cups ⅔ full with batter. Bake about 15 minutes or until tops spring back when lightly touched. Cool in cups on a wire rack 5 minutes. Remove from cups.
Cool completely. Sprinkle with powdered sugar and/or cocoa powder.
Tips: To make 1¼ cups sour milk, place 4 teaspoons lemon juice or vinegar in a glass measuring cup. Add enough fat-free milk to make 1¼ cups total liquid; stir. Let stand for 5 minutes before using.
If using a sugar substitute, choose from Equal® Sugar Lite or Splenda® Sugar Blend for Baking. Follow package directions to use product amount equivalent to 1¼ cups sugar. Nutrition Per Serving with Substitute: same as below, except 151 calories, 20 g carbohydrate.