Melt the butter in a medium saucepan over medium heat until it’s fully liquid.
Whisk in the flour and keep whisking for about 2 minutes, until it bubbles and turns a pale golden color.
Pour in the heavy cream and milk slowly, whisking the whole time so you don’t end up with lumps.
Turn the heat down to low.
Add the cheddar and American cheese, stirring until they melt and the sauce looks smooth.
Mix in the salt and garlic powder so the flavors blend together.
Let it cook for another 2–3 minutes, whisking often, until the sauce thickens up to how you like it.
Take it off the heat and serve right away while it’s warm and creamy.