Using an electric mixer, beat butter, at room temperature, until very pale and smooth. Gradually add icing sugar mixture, beating until well combined. Beat in a little yellow food colouring (optional) to tint.
Using a small, sharp knife, cut, remove and set aside a small, cone-shaped piece from the top of each hot cross buns, leaving a 1cm-wide border. Fill holes using lemon curd. Replace tops, smear with a little buttercream, then sprinkle over coconut flakes.
Cut rainbow sour strap lengthways to separate colours, then cut lengths into small triangles. Fill a piping bag, fitted with a 1.5cm nozzle, with remaining buttercream. Pipe rounds to form rounded cone shapes. Using triangle shapes and candy eyes (2 for each chick), decorate buttercream to create little faces and feet. Decorate chicks with flower icing decorations (3 for each chick).
If you’re not keen on using lemon curd for these easter chicks, substitute it with your favourite jam or spread instead.