Preheat oven to 350F. Line a cupcake pan with 12 liners.

Measure out the flour, add the baking powder, mix well and set aside.

Using a stand mixer or hand mixer, beat the butter and sugar together until light and fluffy, add the eggs one at a time, beating well after each addition. Beat in the oil and vanilla, mix well.

Add in the flour mixture in three additions, mixing well each time.

Divide the batter equally into 5 bowls. Add different food colour to each bowl, mix well. Using about a heaping tablespoon of each colour, begin layering the colours in each cup, tapping down the batter in the pan after each addition. Start with purple, then blue, green, yellow and red. Do not fill the liners more than ¾ full.

Bake the cupcakes for 20-25 minutes until risen and toothpick in center comes out clean. Let the cupcakes cool for a few minutes before removing from pan to a wire rack to cool completely before icing.

To make the buttercream icing, beat the butter until light and fluffy, slowly add in the icing sugar until you have a thick icing, add in the vanilla and use only enough milk to adjust the consistency until it is soft, but still holds it shape. Fill a piping bag fitted with your favourite decorating tip and swirl the top of each cupcake with icing. Decorate with rainbow coloured sprinkles.            

Best eaten immediately or within two days. To store, keep refrigerated.