PREP: Preheat the oven to 325 degrees F. Line a 9x9-inch metal baking pan with parchment paper (leaving an over-hang for easy removal) and grease with cooking spray.
CRUST: In a medium bowl, combine the melted (but not hot!) butter, brown sugar, vanilla extract, and salt. Stir in the flour until completely combined. The dough will be thick. Press this dough firmly and evenly into your prepared pan. (I like to place a piece of parchment paper on top and press down). Bake for 17-23 minutes or until the edges are slightly browned. Remove from the oven and using a fork, poke holes all over through just the top half of the warm crust (do not press the fork all the way through). Let the crust cool slightly while you prepare the caramel.
CARAMEL: Place the cubed butter and brown sugar in a small pot over medium-low heat. Leave mixture alone until the butter is melted, then whisk it together with the sugar until emulsified (butter isn't separating from sugar). Then leave mixture alone until it comes to a simmer. As soon as small bubbles appear, add sweetened condensed milk & salt. Whisk constantly for 5 full 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 into an even layer with a spatula.
BAKE: Bake for 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!)
COOL: Allow the bars to cool at room temperature for 25 minutes, then transfer them to the fridge for another 25 minutes to cool down further. However, make sure the bars aren't cold as this might prevent the chocolate from setting properly.
CHOCOLATE TOPPING: In a microwave-safe bowl, combine the chocolate chips and vegetable oil. Microwave in 30-second intervals, stirring for 20 seconds in between, until the chocolate is fully melted and smooth. Pour this 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.
LET SET: 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 from the pan onto a cutting board. To cut the bars, use a very 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 the slicing easier.
Video
Notes
Note 1:9x9-Inch Pan is Key: This 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:Full-Fat Sweetened Condensed Milk: Use only full-fat; reduced-fat or sugar-free won't give the same results. Avoid accidentally using evaporated milk.Note 3:Choosing Chocolate Chips: Semi-sweet cuts sweetness, but milk chocolate is closer to Twix's taste. High-quality chocolate like Ghiradelli or Guittard melts smoother and tastes better.