Make the Shortbread Base. Preheat your oven to 350°F. Line an 8x8-inch baking dish with parchment paper. In a bowl, cream together butter and sugar until light and fluffy. Add flour and salt, mixing until a dough forms. Press dough evenly into the baking dish. Bake for 18–20 minutes, or until golden around the edges. Allow to cool completely.
Prepare the Caramel Layer. In a saucepan over medium heat, combine butter, brown sugar, corn syrup, and sweetened condensed milk. Stir constantly until mixture begins to bubble, then reduce heat to low and continue stirring for 5–7 minutes until thickened and golden. Remove from heat, stir in vanilla extract, and immediately pour caramel over cooled shortbread. Spread evenly. Let it sit at room temperature for 1 hour or in the fridge for 30 minutes.
Add the Chocolate Topping. Melt chocolate chips with oil in the microwave in 30-second intervals, stirring until smooth. Pour melted chocolate over the caramel layer and spread evenly with a spatula. Tap the pan gently to release air bubbles.
Chill and Slice. Refrigerate bars for 1–2 hours until the chocolate is set. Remove from the pan using parchment paper, then slice into bars or squares with a sharp knife. For clean cuts, warm the knife under hot water and dry it between slices.
Serve and Enjoy. Your homemade Twix bars are ready! Pair them with coffee, gift them to friends, or store them for later snacking.