Spray an 8×8 inch pan with nonstick cooking spray or line with parchment paper. Preheat oven to 350f degrees. Mix together box cake, butter, and eggs with a hand mixer, then fold in chocolate chips.
Spread batter into prepared dish (batter will be thick). Bake for about 15-20 minutes or until top is firm. Let cool completely before icing.
To make frosting, beat together butter, powdered sugar, and milk until thick and creamy. Take out ½ cup of it, and mix with pink gel coloring.
Once bars are cooled, spread white frosting out in a thick layer on top. Drop 4-5 spoonfuls of prepared pink icing all over your surface, and then using a butter knife, swirl all around to give a marbled look.
Cut into squares before serving.