1Heat oven to 350°F (325°F for dark or nonstick pan). Place paper baking cup in each of 12 regular-size muffin cups. Make cake mix as directed, using water, oil and eggs. Divide batter among muffin cups (cups will be full).
2Bake 20 to 25 minutes or until toothpick inserted near center comes out clean. Cool completely in pan on cooling rack. Remove cupcakes from pan, loosening edges with knife if necessary. Remove paper liners. Cut tops off cupcakes; set aside.
3In small bowl, mix powdered sugar, cocoa, 3 tablespoons water and the vanilla until well blended. In each of 4 pint-size jars, place 1 cupcake bottom; top with a spoonful of chocolate filling. Repeat layers. Top with remaining cupcake bottoms. Refrigerate until serving time. Garnish with whipped cream just before serving.
4For each whoopie pie, spread 2 tablespoons frosting on cut side of 1 reserved cupcake top. Place second cupcake top, cut side down, on frosting; gently press together. Store covered at room temperature.