Line 18 sections of 2 x 12-section bun trays with paper cake cases. Put all the cake ingredients in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Beat together the unsalted butter and icing sugar in a bowl until smooth and creamy. Beat in the food colouring. Spread the buttercream over the cooled cakes using a small palette knife. Decorate the cakes with plenty of small sweets and sugar sprinkles, if using.
Arrange a layer of cakes on a serving plate and stack another 2 or 3 tiers on top. Push the required amount of birthday candles and candleholders into the cakes.