Preheat oven to 325°F and spray a 9x13 dish with nonstick cooking spray then place parchment paper in the dish so the edges overhand the dish (this will make removing the cookies after baking really easy).
In a stand mixer with a paddle attachment, cream the butter until nice and smooth. (12 tbs salted butter)
Once butter is creamed add in the powdered sugar and vanilla and mix until combined. Scraping down the sides when needed. (½ cup powdered sugar, 1 teaspoon vanilla).
On a low-speed mix in the flour until combined. (1 ½ cups flour)
Gather the dough together with your hands to form a smooth ball- try not to knead too much. Then press into prepared dish and flatten the top with a rolling pin or cup or jar.
Bake for 20 minutes. The edges with be just starting to turn golden. Remove from oven and carefully but quickly slice into bars and prick with a fork (to make it look like Walkers cookies if you want).
Return to oven and continue baking for 6-8 minutes. The top of the shortbread will be lightly golden. Then turn the oven off and leave the door cracked open and allow the shortbread to come to room temperature.
Once bars are cooled completely, melt your candy melts in the microwave. Make sure to use 50% power and check candy melts every 30 seconds, stir and continue heating until melted and smooth. Then dip the cookies into the chocolate and place on a parchment lined baking sheet until hardened.