In a small saucepan, over medium heat, melt butter. Whisk in milk followed by the sugar and cocoa powder mixture. Whisk continuously, beating out any lumps as they form, until mixture begins to thicken and bubble 1-3 minutes.
Remove from heat and continue whisking for about 30 seconds to cool the sauce slightly before whisking in the vanilla. Allow to sit and thicken for 2 to 3 minutes. Serve over ice cream, brownies, or pancakes and enjoy!