Place sweetened, condensed milk into the top of a double boiler. Cook over simmering water for 1½ to 2 hours, stirring every five minutes, until a golden caramel color. Chill half of mixture. Cool the other half to room temperature.

Heat 2 cups of milk almost to a simmer in a medium saucepan. Whisk eggs in a bowl. Slowly pour about one half cup hot milk into eggs, whisking rapidly to prevent eggs from cooking. Whisk eggs into remaining milk in saucepan. Stir constantly over medium-low heat until custard thickens. It should heavily coat a spoon. Do not allow custard to boil.

Remove from heat and stir in the room-temperature caramel, add the remaining 1 cup milk and the vanilla. Refrigerate overnight or until very cold.

Pour ice cream base into an ice-cream maker and process according to manufacturer's instructions. When ice cream is thick and almost done, dollop in chilled caramel into mixture, allowing it to drip in ribbons from a spoon. Continue processing until done. Freeze if you can wait! Enjoy!