Preheat the oven to 350º Fahrenheit. Lightly grease a 10x15" or 9x13" baking pan. Line with parchment paper so that you can easily remove the bars.
In a bowl, cream the butter and brown sugar until smooth.
Add the egg and vanilla. Mix again, scraping the sides of the bowl as needed.
Add the flour and salt. Mix just until combined. The mixture will be thick and slightly crumbly.
Place the mixture into the prepared pan and press along the bottom to cover it evenly.
Bake for 17 minutes (10x15" pan) or 18 minutes (9x13" pan) until the shortbread is set and just slightly browned around the edges.
When the bars come out of the oven, immediately place the unwrapped Hershey bars on top of the cookie bars. Let them sit for about 5 minutes to melt. The cookie bars should be warm enough to melt the chocolate if you put them on right away. If for some reason they are not melting, place them in the oven (the oven can be turned off...it will still be warm from baking) for about 2-3 minutes to finish melting the chocolate.
Use a spatula to smooth the chocolate evenly.
Immediately sprinkle on any toppings such as holiday sprinkles.
Allow the chocolate to set. This will take a couple of hours.
When the chocolate is still slightly soft, use a sharp knife to cut the bars.
Store in an airtight container at room temperature for 4-5 days. These also freeze well for up to 6 weeks.
The calories shown are based on the bars being cut into 20 pieces, with 1 serving being 1 cookie bar. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**