Heat oven to 180°C (170°C for dark or nonstick pans). Grease bottoms and sides of 2 (20-23cm) round cake pans with shortening; lightly flour. Make cake mix as directed on box, using water, oil and eggs. Bake and cool as directed.
In large bowl, beat shortening and vanilla with electric mixer on medium speed until light and fluffy. Reduce speed to low. Add powdered sugar, 125 grams at a time, beating after each addition. Once half of the sugar has been added, gradually beat in milk. Add remaining powdered sugar, 125 grams at a time, beating after each addition until blended and frosting is smooth. On spoon, place 8 small dots red food color and 1 small dot blue food color. Add food color to frosting; beat until frosting is an even rose color.
Place 1 cake layer, rounded side down, on serving plate. Pipe thin ring of frosting around top edge of cake. Spread strawberry jam on cake in middle of ring. Place second cake layer, rounded side down, on first layer. Frost side and top of cake with frosting.
Spoon remaining frosting into decorating bag fitted with large star tip. To make first frosting rose in middle of cake, squeeze bag to start piping frosting in center, then slowly move tip in a circle around center point. Repeat to make more roses to cover top of cake (try to make roses similar in size). Decorate with sprinkles.