Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line a 20cm round cake tin and a 28cm x 18cm rectangular baking tin or heatproof glass baking dish with baking paper.
Make up the cake mix as directed on the box.
Pour 550g of the cake mix into the rectangular tin or baking dish and the remaining cake mix (about 350g) into the round tin. 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 tins for 10 minutes, then remove from the tins, 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 icing onto one of the 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 icing, using up the rest of the first tub of icing. Chill for 30 minutes.
Spread the remaining tub of icing over the 2 cakes. Chill for 15 minutes.
Twist 3 strawberry laces together and tie at one end. Place the knotted end at the half circle end of the base cake and lay onto the icing. 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.