Add butter, brown sugar, corn syrup, and sweetened condensed milk to a medium pot on medium heat.
Stir constantly until butter is melted and ingredients are completely combined.
Bring mixture to a boil and boil for 1 minute, stirring constantly. Immediately remove from heat.
Let mixture cool for about 2–3 minutes, stirring occasionally. Then add in vanilla and salt to preference. Be sure to use more if you're trying to make a salted caramel sauce.
Let sauce thicken for 10–20 minutes, stirring occasionally, and then serve by transferring caramel to a bowl with your desired toppings (see note 1).
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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.
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.