COMBINE. Mix butter, brown sugar, corn syrup, and sweetened condensed milk in a medium pot on medium heat.
MELT. Stir constantly until butter is melted and ingredients are completely combined.
BOIL. Bring mixture to a boil and boil for 2 minutes, stirring constantly. Immediately remove from heat.
COOL. Let mixture cool for about 2-3 minutes, stirring occasionally, and then add in vanilla and salt to taste. Add salt gradually. (Not all salts will salt this sauce the same. Add more for a salted caramel sauce and less to season sauce.)
SERVE. Let sauce thicken for about 10 minutes, stirring occasionally, and then serve by transferring caramel to a bowl, topping with optional crushed toffee candy, and your choice of dippers. (Note 1)
STORAGE: Store remaining sauce in an airtight container in fridge for up to 5 days; however, it doesn't freeze well. To reheat, warm in a small pan over low heat until heated through, or microwave in short bursts. If sauce is too thick, gradually add heavy cream or milk to thin it slightly.
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Notes
Note 1: Here are some tasty treats to dip in the caramel sauce:
Apples: Granny Smith, Fuji, and Honeycrisp are great choices. You can also try making apple nachos by drizzling sauce over thinly sliced apples and adding crushed toffee bits.
Bananas: Make sure they're not too ripe or they'll fall apart. Dip them in the sauce, then in melted dark chocolate, and top with crushed nuts before freezing.
Other treats: Cubed Rice Krispie Treats, brownie bites, large marshmallows, and pretzels.
Packaged cookies: Oreos, Nutter Butters, Pirouettes, Milanos, Girl Scout Cookies, Graham crackers, and chocolate Graham crackers.
Nutritional information does not include the optional toffee bits.