Preheat oven to 170°C and grease a muffin tin or line with paper cups (I like to use silicone cups).
Melt the butter and set aside.
In a medium bowl, mix flour, baking powder, baking soda and salt.
In a large bowl, beat the sugar and the melted butter until foamy.
Add the eggs, 1 egg at a time, and beat after each addition.
Add the vanilla extract, then beat again.
Alternately, flour and buttermilk, starting and ending with the flour. Don’t overmix.
Add in some colored sprinkles. I used about 4 tablespoons.
Fill up your muffin tin/cupcake liners and bake ca. 15-20 minutes.
Let cool completely before applying the frosting.
Divide the buttercream over 4 bowls and add food coloring to each one. I used blue, red, green and yellow.
You can make the colors as light or dark as you wish. Start with just a few drops and add more if you want a deeper color.
Then lay a piece of cling film on your work space and line the colored buttercream on it. Then roll up and close the ends.
Put in the fridge for about half an hour. You don’t want to leave it for too long as it will firm up too much and you will have to let it come to room temperature again to be able to pipe it.
Prepare a piping bag with the nozzle you want to use. Cut of one end of the cling film and place it in the piping bag.
Pipe on frosting.