Heat oven to 350°F. Grease one 4-cup and one 2-cup glass measuring cup with vegetable cooking spray. Prepare batter according to package directions. Pour 3-1/2 cups batter into larger measuring cup and remaining batter into smaller one. Bake smaller cake 25 to 30 minutes and larger one 35 to 40 minutes, or until a toothpick inserted in centers comes out clean. Transfer cakes to a rack; cool completely.
Spoon 3/4 cup vanilla frosting into a pastry bag fitted with a small plain writing tip (#3) and 1 cup pink frosting into a pastry bag fitted with a plain ribbon tip. Invert cooled cakes onto a cutting board and level tops. Place large cake on a serving platter and place smaller cake on top to form Barbie's skirt. Insert doll in center, arms raised for easier decorating. Trim cake from doll's waist to round off gown shape.
Use vanilla frosting from can to form a wide V-shape down the front of gown. Press small pink flowers over frosting. Spread strawberry frosting over the rest of doll's gown and up her torso. Spread as smoothly as possible. With white frosting in pastry bag, pipe squiggly decorative pattern over gown. Pipe small white dots along front base of gown. With pink frosting from pastry bag, pipe a ribbon edge along bottom of gown. Lower doll's arms and finish top of gown with pink frosting. Use pink gel to make small dots over pink area of gown.
*Instead of strawberry frosting, you may use another can of vanilla frosting. Tint to any pastel color with food coloring.