Line an 8×8″ pan with parchment. Grease the parchment with a little butter & set aside.
Place the butter in a large microwave-safe bowl & heat until melted (approx 15 seconds).
Add the sugar, corn syrup & sweetened condensed milk & stir until smooth.
Return the bowl to the microwave & heat for another 5-1/2 to 7 minutes depending on the wattage of your microwave. (remove & carefully stir at the halfway point)
Test to see if the caramel is ready by placing a small spoon into the caramel & then immediately into a cup of cold water. If it is able to be formed into a soft ball of caramel, then it is done & ready to be transferred out of the bowl. If it is still too soft, cook an additional 30-60 seconds.
Carefully fold in the vanilla, sea salt & then the walnuts if desired.
Pour into the prepared pan & refrigerate for about 20 minutes to cool.
Use a plastic knife to cut (because this prevents stickinand wrap each piece with wax or parchment paper squares.