1Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 24 regular-size muffin cups.
2Make and bake cake as directed on box for cupcakes. Cool in pans 10 minutes; remove from pans to cooling racks. Cool completely, about 30 minutes.
3Spoon frosting into decorating bag fitted with large round tip. Pipe dollop of frosting on top of each cupcake.
4In small microwavable bowl, microwave chocolate chips and shortening uncovered on High 1 minute, stirring once halfway through heating. Drizzle melted chocolate over frosted cupcakes; sprinkle with hearts. Store loosely covered.