Pre-heat oven to 180°C (160°C /Gas Mark 4). Place 12 cupcake cases into a 12 hole muffin tray.
Cream together the butter and sugar until light and fluffy. Gradually beat in the eggs and Vanilla Extract, if the mixture starts to curdle, add a little flour.
Fold in the remaining flour and Baking Powder with a metal spoon.
Divide the mixture between the baking cases and bake for 20-25 minutes until well risen and firm to the touch. Remove from oven, leave to cool on a cooling rack.
Divide the buttercream equally between 5 bowls. Add a few drops of Pink Colour Gel into one bowl of buttercream and mix through, adding a few drops of colour until you have a bright pink buttercream. Repeat this step for each colour until you have 5 different coloured bowls of buttercream.
Place each colour buttercream into a separate piping bag. Lay a piece of cling film onto your work surface and pipe a strip of each colour buttercream onto the cling film, starting with pink then orange, yellow, green and blue. – make sure the lines of buttercream are not too thick and they should be touching each other.
Once you have piped a line of each colour buttercream roll the cling film to create a sausage shape. Cut the end of the cling film to reveal the buttercream at one end. Place the cling film into a piping bag fitted with a star nozzle.
Once the cupcakes are cooled pipe a buttercream swirl onto the top of each cupcake to create the rainbow buttercream swirl.