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. 

Place the butter in a freestanding mixer bowl and beat until smooth and glossy. Add the icing sugar in 4 intervals, making sure it is fully incorporated after each interval. Once ready, 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.