Preheat oven to 350 degrees F. Grease a 9 x 9-inch pan and set aside.
In a large mixing bowl, beat together the butter, powdered sugar, vanilla, and salt until the mixture looks like a coarse sand. Mix in the flour until the dough comes together.
Press the dough evenly into the prepared pan and bake for 20 minutes, or until the surface of the shortbread looks completely dry. Cool in pan for 15 minutes.
Assemble the Twix Bars
In a microwave safe bowl, microwave the caramel candies until completely melted and smooth, about 1-1 1/2 minutes. Using an offset spatula, spread the caramel evenly over the shortbread layer. Allow to cool for 15 minutes to set.
Turn out shortbread onto a cutting board and cut into 9 1-inch wide pieces. Then, in turn, cut those pieces in half, creating 18 1-inch wide and 4 1/2-inch long candy bars.
In another microwave safe bowl, melt the chopped chocolate for 15 seconds at a time, stirring between each interval, until smooth. Dip each candy bar into the chocolate, remove any excess chocolate, and set on wax paper to set completely (about 1 hour). Store in an airtight container at room temperature.
I'm a student, photographer, and science teacher with a sweet tooth. As a self-taught baker, my love affair with pastries began in my own kitchen. The voice behind the blog Pastry Affair, I develop recipes for the home baker, combining a mixture of rusticity, homespun flavors, and honesty.