Make the candied peanuts:
Place a sheet of parchment paper on a cookie sheet and lightly spray with cooking spray. Set aside.
In a medium non-stick skillet over low heat, melt the butter. Add the granulated sugar followed by the peanuts and splash of vanilla extract. Stir to coat the peanuts. Increase the heat to medium and cook for about 5 minutes stirring THE ENTIRE TIME until all the sugar is melted, caramelized, and the peanuts are well coated with sugar.
Transfer the candied peanuts immediately to the sheet of parchment paper. With a spatula and moving quickly (quickly is key, otherwise the peanuts will cool into a big clump) separate the peanuts as best you can. Allow them to cool completely, and then chop them coarsely. Set aside in a bowl until ready to use.
Make the shortbread layer:
Preheat oven to 350 degrees F (177 degrees C). Line an 8x8 inch square pan with parchment paper, leaving an overhang. Set aside. In a medium bowl combine flour and salt. Set aside.
In the bowl of an electric mixer (hand mixer works too) fitted with a paddle attachment over medium/high speed, cream the butter and sugar until light and fluffy, about 2 minutes. Add in the vanilla. Reduce the speed to low and add the flour/salt mixture and mix until just combined, about 8-10 seconds or so. Spoon the dough in your prepared pan and press it evenly with your hands or using an offset spatula. The point is to cover the pan as evenly as possible. Place the pan in the oven and bake for about 20 minutes, or until pale golden in color. Remove from oven and place on a wire rack to cool.
Make the chocolate topping:
Place chocolate and butter in a stainless steel bowl set over a sauce pan half filled with simmering water over low heat. Melt the chocolate, stirring occasionally. When chocolate is fully melted, turn off the heat but leave the bowl over the sauce pan while you assemble this situation.
To assemble:
Sprinkle the chopped candied peanuts over the cooled shortbread layer. Over the peanuts spoon the dulce de leche and even it out with a small offset spatula or butter knife. Make sure to cover the peanuts as best you can. Pour the melted chocolate evenly over the dulce de leche and either leave out at room temperature to set or place in the freezer for about 15-20 minutes. Once set, lift the parchment overhang and place the square on a cutting board or a flat surface. Cut into squares using a sharp knife.
Store the shortbread squares in the refrigerator to keep the chocolate nice and firm. Make about 16 squares.