Prepare two large baking sheets lined with parchment paper.
Add butter, sugar, and karo syrup to a large heavy-bottom saucepan over medium heat. Stir over medium heat until mixture begins to boil, about 5-10 minutes.
VERY slowly add the evaporated milk, taking about 12-15 minutes to slowly add it, while stirring constantly. You want to make sure the mixture maintains a constant boil, otherwise your caramel can curdle.
Stir the mixture constantly, scraping the sides occasionally until it reaches the soft ball stage (about 235 degrees, if using a candy thermometer). It takes patience and time, but it's sooo worth it!
I don't always trust or rely on a candy thermometer--I like to test it the old fashioned way. Drop a spoonful of hot caramel sauce into a cup of ice water and mold it with your fingers into a ball. When ready it will feel pliable and should be sturdy enough to mold in your hands, in the ice water. If you're not sure if it's too soft, err on the side of cooking a few extra minutes.
Once you reach 235 degrees F / or the soft ball stage, remove from heat. Stir in vanilla.
Dip Pretzel Rods in caramel. I use a spoon to scoop the sauce onto the pretzels and turn them as I go. Allow excess sauce to drip off and place the dipped pretzels on a parchment lined baking sheet.
After the caramel sets and cools some of it may pool at the base of the pretzel, causing the bottom to be flat. Use your hands to mold the caramel back around the rod (or just roll the rod gently on the parchment paper to smooth it around).
Melt chocolate in a microwave-safe bowl, on half power, stirring every 30 second, until smooth.
Pour the chocolate into a tall glass. Dip the caramel-coated pretzels in melted chocolate. You can use a spoon or spatula to help smooth the chocolate up and around if you need it.
Lay the dipped pretzels on a parchment lined sheet. At this point you could add other toppings like mini chocolate chips, sprinkles, crushed heath bar etc.
If any chocolate pools at the base of the pretzels as they set you could easily cut it off with a knife, if you want it to look prettier..
You can store the pretzels in the fridge in a covered container, if you want. Let the pretzels sit at room temperature for 5-10 minutes before eating.
Tips for making, storing, and gifting these pretzel rods are listed in the post above!