If you decorate the card with TV characters, please make sure that you only use CBeebies characters! With the base cake and the submarine covered in fondant now is your time to get creative with fondant decorations!

Cut a small heart shape out of the baby blue ready-to-roll icing, brushing the cut edges with your finger to soften these. Place the heart into position. This cake is made from two large sponges cut into two layers each, so there are four layers inside the cake. Make each sponge cake one at a time with 4 large eggs, 225g unsalted butter, 225g caster sugar, 225g plain flour, 2½ tsp baking powder and the zest of 2 lemons (plus the juice of 1) in each cake. You might want to make the sponges in advance, in which case wrap them well in cling film to prevent them drying out. Using a small amount of green gel food colouring, colour 1kg of fondant to cover the cake. Roll this out to a circle of about 40cm diameter and use your rolling pin to carefully lift this onto the cake. Use a cake smoother (or your hands) to smooth the fondant onto and all around the cake. Cut the excess fondant from the base of the cake with a sharp knife. Using a small amount of blue gel food colouring, colour 1kg of fondant to cover the cake. Roll this out to a circle of about 40cm diameter and use your rolling pin to carefully lift this onto the cake. Use a cake smoother (or your hands) to smooth the fondant onto and all around the cake. Cut the excess fondant from the base of the cake with a sharp knife.To make the buttercream, beat the butter in a large bowl with an electric hand whisk or in a stand mixer. Pour in the icing sugar, add the vanilla paste and mix slowly on the lowest setting on your mixer until completely incorporated. Repeat until it is a four layer sponge cake (sponge, buttercream, jam, buttercream, sponge, etc.). Coat the entire cake in a thin layer of buttercream and smooth with an offset palette knife. Colour small amounts of fondant icing with all the colours you will need and wrap each ball in cling film to prevent it from hardening or drying out.

The submarine is made from two pieces of cake. I searched online for ‘Octonauts Gup-A’ and there were loads of images to guide the shape. The bottom cake should be carved carefully with a serrated bread knife into a teardrop shape with a slight curve on the underside. The top cake should be carved into a triangular shape with a slight curve to the top. Sandwich the two layers together with buttercream and add a light crumb coat of buttercream around both cakes.

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Fill the hollow with the coloured sweets. With my version, I separated out all of the yellow sweets to put these at the top to look more like treasure. Put gold coins on top of these. Using curved strips of yellow and green fondant, wrap the base of the cake to give the effect of ocean contours and the sea bed. Attach these to the sides of the cake with a little buttercream.