Mix cake following box instructions and bake in two 12" square cake pans; let cakes cool completely.

Cut out onesie shapes with cookie cutter and place in refrigerator for 10 minutes.

Make frosting by beating powdered sugar, butter, vanilla extract and milk until completely smooth, scraping down sides of bowl often (you can purchase icing to save time).

Ice cake with "crumb coat" by applying thin layer of frosting evenly over cakes

place in freezer for 15-30 minutes; add second layer of frosting and harden cakes in refrigerator for 10 minutes.

Color fondant with food coloring, adding in small increments until desired color is achieved.

Roll out about 1 cup of fondant to ¼" thick on counter lightly dusted with powdered sugar.

Drape over cake and trim around edges with pizza cutter; repeat with each cake.

Detail cakes with fondant utensils.

Use small end of piping bag tip to make rings for buttons.