Preheat oven to 350 degrees. Line standard 12-cup muffin tin (or 6-cup muffin tins) with paper liners. In a large mixing bowl, combine flour, sugar, baking powder, baking soda, and salt; set aside.
With an electric mixer, combine cocoa and 3 tablespoons hot water until a thick paste forms (this process intensifies the chocolate flavor). Add butter, buttermilk, egg, and egg white; beat until combined. Whisk in flour mixture until smooth.
Scoop (or spoon) batter into prepared tin(s). Bake until a toothpick inserted in the center of a cupcake comes out clean, about 20 minutes. Transfer cupcakes to a wire rack to cool completely.
For the light cream-cheese icing: With an electric mixer, beat 3 ounces softened reduced-fat cream cheese and 1 cup confectioners' sugar until light and fluffy. (To make without a mixer, sift sugar before stirring into cream cheese).Use an offset spatula or butter knife to spread icing over tops. Decorate as desired.