Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease and line 3 x 19cm cake tins with baking parchment, allowing a 2.5cm ‘collar’ to sit above the rim of each tin.
Add the butter and sugar to a large mixing bowl. Using a hand-held electric whisk, beat together until light and fluffy. Keep the beaters going, and add in the egg a little at a time, making sure it is fully incorporated after each addition. When all the egg is incorporated, stir in the vanilla extract and sift in the flour and baking powder. Carefully fold together until you have a smooth mixture with no lumps.
Spoon 2 tablespoons of the cake mix into each of the ice-cream cones, transfer to a baking tray and set aside. Divide the remaining cake mixture between the prepared tins and bake for 50 minutes or until well risen and golden, and the tops of the cakes spring back when lightly pressed. Leave to cool in the tin for 5 minutes, then turn out (peeling off the lining paper) onto a wire rack to cool completely.
While the cakes are cooling, bake the ice-cream cones for 20 minutes. Remove from the oven and leave to cool alongside the cakes on the wire rack.
Decorate the cones and cake. First, make the buttercream. In a large bowl, beat together the butter and icing sugar until fluffy.
Next, slice the tops off the cone cakes so that you have a level surface. Fit the icing bag with the star-shape nozzle, and spoon enough buttercream into the bag so that it is ¾ full. Pipe the buttercream onto the top of the cone cakes to create an ice-cream effect. Decorate the cones with the flake, writing icing and coloured vermicelli strands. Squeeze the remaining buttercream back into the bowl.
Trim the tops of the cakes to create a flat surface, then sandwich together with thin layers of buttercream and the jam. Spread the remaining buttercream over the tops and sides of the main cake.
Knead then roll each piece of the blue icing out to a rectangle measuring 30cm x 10cm – see Cook’s tip. Using a cocktail stick, score a wave shape onto the long side of each piece. When you’re happy with the shape, cut it out using a sharp knife. Press the waves around the base rim of the cake, making sure the seams join at the sides of the cake – the buttercream will help it to stick in place.
Scatter and press the crushed biscuits over the cake to create a sand effect. You will need to gather the ‘sand’ several times from under the wire rack and re-apply until it is completely stuck to the cake. Carefully transfer the cake to a cake plate or board.
To make the napkin, roll 50g of the white ready to roll icing out to a 12cm x 12cm square. Press the back of a fork around the edges to create the effect of a napkin then position on top of the cake.
Use the remaining ready to roll icing to cut out ocean-themed shapes – we made seaweed, fish, octopus, starfish, shell and pebble shapes, but you could make anything you like. Draw details onto the shapes using the dark chocolate writing icing and the point of a cocktail stick. Squeeze the white chocolate writing icing onto the backs of each shape and leave for 2 minutes – this makes it stickier so it will hold better on the cake. Stick the shapes to the blue ‘sea’ icing. The seaweed shapes can be used to cover the seams.
To finish, insert the 3 skewers into the ‘napkin’, then push an ice-cream cone cake onto each skewer to secure it in place.
Cook's tip: bake the cone cakes and main cakes separately to avoid opening the oven door partway through cooking. Knead the ready to roll icing before rolling it out – you’ll find it easier to roll and shape. When making your decorations, keep a ruler to hand so you can measure how long and wide to make the icing.