Line an 8x8-inch baking pan with parchment, leaving overhang for easy removal.

In a medium saucepan over low heat, stir chocolate chips, condensed milk, and butter, until smooth.

Remove from heat, stir in vanilla extract.

Pour into the pan, drop spoons full of caramel on top, then swirl into the chocolate with a knife.

Sprinkle it with sea salt.

Refrigerate 3-4 hours until set. Lift out and cut into squares.