In microwave safe bowl ,melt the chocolate chips until just melted 1-2 minutes.
Stir with a spatula or scraper. Stir in the butter. Let the mixture sit for 4-5 minutes.
Add the can of sweeetened condensed milk and stir until smooth.
That's it! Frost the brownies or cake immediately as this mixture will set up and become more fudgey as it cools. If it cools too fast for you, place in the micrewave for 15 seconds to loosen it back up a bit.