Preheat oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Well grease 3x8-inch round cake tins.
Mix the cake mix, oil, water and eggs gently together in a large bowl. (Both boxes can be made at one time; do not increase whisking time.) Whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy.
Spread one-third of the batter in each of two cake tins (about 525g each). To the remaining one-third of the batter, whisk in cocoa powder until well blended. Whisk in food colouring to reach desired level of red. Pour batter into third tin.
Bake in the centre of the oven for 24-28 minutes or until a rounded knife inserted into the centre of the cake comes out clean. Leave in tins for 10 minutes. Remove from tins to cooling rack. Cool completely, about 1 hour. Trim off rounded tops of cake layers if needed.
Add a dab of icing in the centre of the cake plate to keep the cake in position. Place first vanilla cake layer on plate and ice with 140g of the vanilla icing. Place red cake layer on top of first and spread 140g of the icing. Top with last vanilla cake layer. Ice top and sides with thin layer of icing to cover cake. Place in refrigerator for 30 minutes to harden.
Fit a piping bag with a medium star nozzle. Fill bag with icing. Starting about 1 1/2 inches from bottom edge of cake, pipe centre of rosette flower by touching nozzle tip to side of cake to attach icing. Apply steady pressure to bag while squeezing icing and pipe in a tight circle to make a flower about 2 inches in diameter. Tap nozzle to cake to complete flower and lift off. Continue piping flowers to cover side of cake, staggering up and down sides so no empty spaces show. Refill piping bag as needed. Pipe rosette flowers on top of cake to cover.
Scatter heart shape sweets on top of cake. Chill 30 minutes before serving.