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Preheat the oven to 350°F. Lightly grease a 9" x 13" pan and line the pan with a parchment paper sling.
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To make the crust: In a mixing bowl or bowl of a stand mixer fitted with the flat beater, combine the butter, sugar, vanilla, and salt. Beat until smooth, about 3 to 5 minutes.
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Scrape down the sides of the bowl, then add the flour and continue beating just until crumbs start to form and the mixture is somewhat shaggy.
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Transfer the crust mixture to the prepared pan and shake gently to evenly distribute it.
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Firmly press the crust mixture into an even layer with your hands. If the crust mixture becomes too warm and begins sticking to your hands, transfer the pan to the refrigerator to chill for 5 to 10 minutes, then continue pressing.
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Once the crust has been pressed into the pan, chill it for 15 minutes in the freezer or 30 minutes in the refrigerator.
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After the crust has chilled, bake it for 20 to 25 minutes, or until golden brown. Set aside to cool while you make the caramel layer.
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To make the caramel layer: In a large pot, combine the sweetened condensed milk, sugar, butter, and salt. Cook over low heat until the butter is melted and the mixture is smooth.
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Increase the heat to medium and cook, stirring constantly to prevent any burning, until the mixture comes to a boil. After a few minutes, it will thicken slightly, darken in color, and begin to pull away from the sides of the pot; the temperature should be between 225°F and 240°F when measured with a digital or candy thermometer.
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Remove the pot from the heat and stir in the vanilla. Immediately pour the caramel over the crust, spreading it all the way to the edges. (A small offset spatula is a helpful tool here).
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Allow the caramel layer to set up and cool completely either at room temperature or in the refrigerator. At room temperature, the caramel shortbread will take about an hour to cool; in the refrigerator, about 25 minutes.
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To make the chocolate glaze: Place the chocolate in a medium heatproof bowl. Bring the cream to boil, either in a small saucepan on the stovetop or in a heat-proof bowl in the microwave.
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As soon as the cream starts to boil, immediately pour it over the chocolate. Let the chocolate and cream mixture sit undisturbed for 2 minutes before adding the vanilla and then whisking until smooth.
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Pour the chocolate glaze over the caramel layer, spreading it all the way to the edges. Allow the chocolate glaze to set for about 20 to 30 minutes, then use a warm chef’s knife to cut into 32 bars (an 8 x 4 pattern works well here). Serve at room temperature.
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Storage information: Store any leftover chocolate caramel shortbread in an airtight container in the refrigerator for up to a week; freeze for longer storage.
