Small amount of fondant in pink, orange, blue and white
Black edible paint or pen
Preheat the oven to 160°C / 140°C fan / gas 3. Line two baking trays with baking paper and fit two piping bags with 1cm plain nozzles. Sift the icing sugar and ground almonds into a bowl. In a separate bowl, whisk the egg whites, salt and 3 tablespoons of the caster sugar until the mixture forms soft peaks. Whisk in the remaining caster sugar gradually until thick and glossy.
Fold in the icing sugar and almonds until shiny and falling off the spoon or spatula in ribbons. Split between two bowls and colour one batch pink and one orange. Use a small amount of mixture to stick the corners of the baking paper onto the baking trays (this makes the macarons easier to pipe). Transfer to the piping bags and pipe 3cm blobs onto the lined baking trays keeping a 3cm gap between each one. Tap the baking trays onto the work surface to release any bubbles and even out the macarons. Use a damp finger to smooth down any peaks and leave to stand at room temperature for 15 minutes. Bake for 15 minutes or until the macarons come away from the paper easily without sticking. Leave to cool on the baking trays.
While the shells are cooling, make the buttercream and decoration. To make the buttercream, beat the butter, icing sugar, milk and vanilla until pale and creamy. Divide between two bowls and colour one batch pink and one orange.
Make 4 pairs of legs and a pair of pincers for each crab in either pink or orange. For the orange crabs, make a pair of eyes and a white mouth using the white fondant. For the pink crabs, make pairs of blue glasses with white eyes using the blue and white fondant.
Sandwich together the shells with the coloured buttercream, push the legs and pincers into the buttercream centres and add the features with a small amount of buttercream. Finish by drawing on black eye balls and a mouth for the pink crabs and eyes and a moustache for the orange crabs.