For the Shortbread Base:
Preheat oven to 350 degrees (F). Line an 8×8-inch baking pan with parchment paper, allowing two of the sides to overlap. Spray the parchment – and any exposed pan – with non-stick baking spray; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, beat the butter, vanilla extract, confectioners’ sugar, and salt until light and fluffy; about 2 to 3 minutes. Gradually beat in the flour, mixing just until combined. The mixture will be a little crumbly.
Dump the mixture into the prepared pan and spread (or press firmly, if need be) it into an even layer. Bake for 14 to 15 minutes, or until lightly browned. Set aside to cool while you prepare the fudge topping.