This is the best caramel dip recipe ever. Made on the stove with butter, sugar, and sweetened condensed milk, it's the perfect consistency for dipping pretzels and apples.
In a medium saucepan, melt butter over medium heat.
Add the brown sugar, corn syrup, and sweetened condensed milk, stirring to combine.
Stir constantly until mixture comes to a light bubbly boil, with just a few bubbles popping through the surface of the caramel (see notes for more details).
Remove from heat right away and add the vanilla. Stir to combine.
Serve warm or at room temperature with apple slices and pretzels, or drizzle over ice cream.
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Notes
The goal for this dip is to be dip-able, so you'll want to bring the sugar mixture to a nice bubble but not a full boil. If you boil it too long or hard, you'll end up with a thicker caramel that is great for dipping whole apples but not for dipping little apple slices and pretzels.
The corn syrup is a necessary ingredient so please don't try any substitutes! (See my note above for more details, as well as this article from Serious Eats.)
Don't wander away from this recipe while it is cooking. A lot of stirring needs to be done so the bottom of the dip doesn't scorch and burn.
If you do see that the bottom is starting to scorch (you'll see little flecks of really dark sugar when you stir), remove the pan from the heat right away. If the caramel hasn't finished cooking, you can return it to the heat while carefully stirring the top of the caramel and avoiding scraping up the burnt sugar on the bottom of the pan. If the scorch isn't too bad, you won't be able to taste it, but you still might have little flecks of dark sugar in your caramel. It's not ideal, but it's better than feeling like you ruined the whole batch.