Mix together condensed milk, egg yolk and butter (no need to melt the butter)

Put on the microwave for about 1 minutes (to melt the butter) and stir well

Put back in the microwave and cook for about 4 minutes. The condensed milk will start to boil and go up (just like boiling milk) when the condensed milk goes back to the original size, it's ready.

Stir it well and add it to your cake/cupcake or wherever you decide to use it.

Make sure you have a deep enough recipient to cook the condensed milk. Otherwise it will overflow and make a mess in your microwave.

You can also do this on the stove. Just make sure you are constantly stirring, otherwise your condensed milk won't be smooth.

This is a cake recipe where I used this frosting: Marsala Cake