At first, you might think that making a cake look like a swimming pool involves lots of complicated cake carving. Happily, provided that you have the correct pan, the construction of your swimming pool cake isn't anywhere near that hard. A 20 cm (8 inch) or larger shortcake pan naturally provides you with the cake shape you need to support the pretend water in your pool. A shortcake has an indentation in its top that is surrounded by an edge of cake, meant to hold creamy fillings and berries -- and also ideal for pool "water." Use either a cake recipe or cake mix.
Preheat the oven to the temperature specified by the cake recipe you are using. Brush the interior of the shortcake pan with melted butter, including the entire ring around the outside and all the crevices. Tilt the pan around under bright light so you can see whether you've missed any spots.
Shake a little flour around inside the shortcake pan, until all the butter is coated. Turn the pan upside down over a garbage can and gently tap it to remove any excess flour.
Pour the cake batter into the shortcake pan and bake according to the recipe's instructions. The cake is done when a toothpick inserted in the middle comes out clean and the edges pull away slightly from the pan. Remove the cake from the pan and cool it completely on the wire rack.
Mix the gelatine according to package instructions. Pour the gelatine mixture into the indentation on top of the cake, up to the top. Do not overfill.
Chill the cake, with gelatine in place, in your fridge for the time specified in the gelatine instructions. This will allow the gelatin to set, creating the look of smooth blue water in a swimming pool.
Ice the sides and top edge of the cake with white icing. Decorate the sides with coloured gel icing in whatever patterns you like.
Whether you use a boxed cake mix or a cake recipe for your batter, most cake recipes yield enough batter for a standard 22.5 cm (9 inch) cake. Boxed mixes yield the same. You may have a little leftover batter, which you can either discard or bake in cupcake papers in a cupcake pan so you can taste-test the finished product without ruining your hard work. It is extremely important that you let the cake cool to room temperature before adding the liquid gelatine mixture. If the cake is still warm, the gelatine mixture might not set properly, and might instead absorb into the cake. Consider different types of sweets as pool decorations. Round sweets with holes in their middles look like inflatable pool toys, especially if they are brightly coloured.