Combine the condensed milk and sugar in a medium size saucepan and bring to a simmer while stirring over medium heat. Reduce to a low simmer and cook, whisking or stirring constantly, for about 5 minutes, until the sauce has thickened. Remove from the heat and stir in the butter and vanilla. Drizzle the sauce over the dessert of your choice, while it is still warm. Enjoy!
Be sure to drizzle the sauce over your cake while it is still warm. It will harden slightly as it cools and it will form a lovely crust in just minutes.