Preheat oven to 325°F. Line a 9x9-inch metal baking pan with parchment paper (leaving an overhang for easy removal) and grease with cooking spray.
In a medium bowl, combine melted (but not hot!) butter, brown sugar, vanilla, and salt. Stir in flour until completely combined. The dough will be thick. Press dough firmly and evenly into your prepared pan. (I like to place a piece of parchment paper on top and press down). Bake 17–23 minutes or until the edges are slightly browned. Remove from oven, and using a fork, poke holes all over just the top half of the warm crust (do not press the fork all the way through). Let crust cool slightly while you prepare the caramel.
Place cubed butter and brown sugar in a small pot over medium-low heat. Leave mixture alone until butter is melted, then whisk it with the sugar until emulsified (butter isn’t separating from sugar). Leave mixture alone until it comes to a simmer. As soon as small bubbles appear, add sweetened condensed milk and salt. Whisk constantly for 5 minutes or until big slow bubbles start appearing. Remove from heat and whisk for an additional minute before stirring in the vanilla extract. Pour this caramel over the crust and spread it out in an even layer with a spatula.
Bake 12–17 minutes or until the caramel develops a bubbly surface all over and exhibits only a slight jiggle when the pan is lightly shaken. This indicates that the caramel has set. (See picture!)
Allow the bars to cool at room temperature for 25 minutes, then transfer to fridge for another 25 minutes to cool further. Make sure the bars aren’t cold, as this might prevent the chocolate from setting properly.
In a microwave-safe bowl, combine chocolate chips and vegetable oil. Microwave in 30-second intervals, stirring for 20 seconds in between, until chocolate is fully melted and smooth. Pour melted chocolate over the caramel layer and spread it out smoothly with a spatula. To ensure a flat surface, gently tap the pan a few times.
Refrigerate the bars until they’re fully set, about 1 hour. Before cutting, let the bars stand at room temperature for about 10 minutes. Use the parchment paper overhang to remove the bars. To cut them, use a sharp knife that’s been warmed under hot water (dry the knife with a towel after each warming). Repeat this process with each cut to make slicing easier.
Video
Notes
Note 1: A 9x9-inch pan is important for success. An 8x8 can work but requires careful caramel monitoring for tiny bubbles and minimal jiggle when pan is shaken. Too much wobble means it’s not set yet, so keep it in the oven a bit longer.Note 2: Use only full-fat sweetened condensed milk; reduced-fat or sugar-free won’t give the same results. Avoid accidentally using evaporated milk.Note 3: Semi-sweet chocolate chips cut sweetness, but milk chocolate is closer to Twix's taste. High-quality chocolate like Ghiradelli or Guittard melts smoother and tastes better.Storage: Store Twix bars at room temperature in an airtight container for 3–5 days. On hotter days, refrigerate to keep layers from melting. For best flavors, serve at room temperature.