First, melt the butter. You can do this a few ways. I melt butter in the microwave heating it for 10 seconds at a time. You could also melt the butter on the stove over medium heat.
Whichever way you choose, make sure to cut the butter into pieces so it melts faster and watch it carefully it burns fast! In the microwave it pops then you have a mess to clean up!
Next, pour the melted butter into the milk, and stir well.
If you used low or skim milk your substitute will be thinner then normal. You can add in some flour (about a tablespoon) to thicken it up. Use this substitute in places of 1 cup of heavy cream in your favorite recipes.