Instructions

• Preheat oven to 300°F. Line a 9×13" pan with parchment paper, leaving some overhang to lift the bars out later, or spray the pan well with non-stick cooking spray.
• Make the shortbread crust: in a large bowl, combine 1 cup butter (cold), 1 cup powdered sugar, 2 cups all-purpose flour, and 1 teaspoon vanilla extract. Use a pastry blender, fork, or your fingers to work the butter into the dry ingredients until the mixture comes together into a cohesive dough.
• Press the shortbread dough evenly into the prepared 9×13" pan, making sure the surface is level and the edges are compacted.
• Bake the crust at 300°F for 28–35 minutes, or until the crust is very lightly golden. Remove from the oven and allow the crust to cool completely in the pan before adding the caramel.
• Prepare the caramel: in a medium, heavy-bottomed saucepan combine 1 cup butter, 1 cup light brown sugar, 1 cup light corn syrup, and 14 ounces sweetened condensed milk.
• Place the saucepan over medium heat and stir constantly until the mixture comes to a steady rolling boil. Be careful—the mixture will bubble vigorously.
• Maintain medium heat and continue stirring constantly. Cook the caramel until it reaches the soft-ball stage, about 15–20 minutes. To test: drop a small spoonful of the hot caramel into a cup of ice water; if you can gather it into a very soft ball with your fingers, it’s ready. Repeat the ice-water test every few minutes once the caramel begins to darken to avoid overcooking.
• When the caramel reaches the soft-ball stage, remove the saucepan from the heat and immediately pour the warm caramel evenly over the cooled shortbread crust. Let the caramel sit at room temperature a few minutes, then refrigerate the pan until the caramel is firm, at least 2 hours or overnight.
• When the caramel is set, prepare the chocolate topping: place 3 cups chopped milk or dark chocolate and 1 Tablespoon butter in a heatproof bowl.
• Melt the chocolate and butter until smooth using a double boiler (set the bowl over simmering water, stirring frequently) or in the microwave at low power in 30-second intervals, stirring between intervals until fully melted and smooth.
• Pour the melted chocolate over the chilled caramel layer and spread into an even layer. Return the pan to the refrigerator and chill until the chocolate is fully set.
• Once set, lift the bars from the pan using the parchment overhang and cut into bars or squares. Store the finished Twix bars covered in the refrigerator for up to 1 week.