Preheat the oven to 180°C (160°C fan forced). Grease and line a 20cm round cake pan and a 28cm x 18cm rectangular baking pan or heatproof glass baking dish with baking paper.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Make up the cake mix as directed on the box, adding the oil, water and eggs.
Pour 550g of the cake mix into the rectangular pan or baking dish and the remaining cake mix (about 350g) into the round pan. Level the surface and bake the cakes for 30 minutes, or until a skewer inserted into the middle of both comes out clean. Leave to cool on wire racks in the cake pans for 10 minutes, then remove from the pans, peel off the baking paper and place onto the wire racks to cool completely.
Cut the round cake in half vertically to create 2 half circles. Trim the rounded tops of each cake to make them level, if necessary. Spread 2 tablespoons of the frosting onto one of the semi-circle cakes and top with the other one to create a double height semi-circle of cake.
Trim the rectangular cake so that it is 8cm wide at one end and 18cm wide at the other end. On a large board, place the larger cake piece with the 8cm end at one end of the board. Place the stacked rounded cake at the 18cm end of the other cake. The half circle cake will be where the popcorn will be placed. Cover the cakes with an initial layer of frosting, using up the rest of the first tub of frosting. Chill for 30 minutes.
Spread the remaining tub of frosting over the 2 cakes. Chill for 15 minutes.
Twist 3 strawberry bootlaces together and tie at one end; or see tip, below. Place the knotted end at the half circle end of the base cake and lay onto the frosting. Trim for the correct length. Repeat with the remaining laces until you have 5 red strips on the ‘popcorn bag’ cake.
Write "Happy Birthday" on the white chocolate piece with the blue writing icing and place onto the centre of the popcorn bag cake. Press the popcorn onto the half circle cake until fully covered.