Bring milk and cream to a boil in a medium saucepan over moderate heat. Remove from heat and whisk in 1 cup dulche de leche until dissolved.
Whisk in vanilla extract and let the mixture cool completely. Chill for at least 4 hours or overnight.
Freeze in ice cream machine until it is almost firm. Fold in the pecans, if using. Transfer ⅓ of the ice cream into a freezer safe ice cream container, swirl a few tablespoons of the reserved dulce de leche, followed by ⅓ of the ice cream and another layer of dulce de leche. Repeat until all of the ice cream and dulce de leche are used up. Freeze.