Stir the shortbread crumbs and sugar together in an 8 x 8 square baking pan. Add the melted butter and stir to combine.
Pat the mixture firmly into the pan. Use a spatula, your hand, or the bottom of a glass to press it into the pan.
Bake the crust for 20 minutes to a light golden brown. Cool the crust in the refrigerator for at least 20 minutes or until it is firm. (Do not place a warm or hot Pyrex dish in the freezer).
Melt the caramel bits and milk in the microwave in 30-second intervals, stirring in between, until it is smooth and creamy.
Immediately spread the caramel mixture evenly over the crust. Be careful not to tear the crust.
Cool the caramel layer in the refrigerator for at least 20 minutes or in the freezer for 10 minutes, or until it is firm.
Melt the chocolate chips and shortening in the microwave for 30 seconds, then stir. Continue to melt in 10-second intervals until it is smooth and creamy. Immediately spread the chocolate evenly over the caramel layer.
Let the chocolate dry for a couple of minutes, then sprinkle the top with flaky sea salt. Place the pan in the refrigerator and let them cool completely.
To neatly cut Million Dollar Bars, remove the pan of bars from the refrigerator and let it sit out for about 5 minutes. This will allow the chocolate to soften just a bit so it doesn’t crack when cut.
Use a sharp knife to cut them into squares or rectangles, to resemble Twix bars.
Store in an airtight container in the refrigerator for up to one week or in the freezer for up to three months.