Preheat oven to 350°F. Mix cake mix, coffee, oil, egg whites and 1 teaspoon of the espresso powder in large bowl with electric mixer on low speed 1 minute. Beat on medium speed 2 minutes. Spoon batter into 24 paper-lined muffin cups, filling each cup 2/3 full.
Bake 18 to 20 minutes or until toothpick inserted into cupcake comes out clean. Cool in pans on wire rack 5 minutes. Remove from pans; cool completely.
Mix water, vanilla and remaining 2 teaspoons espresso powder in small bowl until dissolved. Beat butter and salt in large bowl with electric mixer on medium speed until light and fluffy. Stir in espresso mixture until blended. Gradually add confectioners' sugar, beating until well blended after each addition.
Make an indentation in the center of each cupcake using the handle of a wooden spoon or a straw, making sure not to break through bottom of cupcake. Spoon 1/2 of the frosting into resealable plastic bag. Cut a small piece off one of the bottom corners of bag. Pipe frosting into each cupcake. Spread top of cupcakes with remaining frosting.