For the shortbread:
First, make the shortbread. Preheat the oven to 325 degrees. Generously grease a 9 x 9 baking pan with butter. In the bowl of an electric mixer, combine the butter, salt, and sugar and whip on medium speed until the butter becomes fluffy and white - about 5 minutes. Turn the mixer to low and add the flour and cornstarch. Mix to incorporate. Pat the dough evenly into the pan. Bake for about 30 minutes, until the edges just begin to darken. The shortbread will remain white. Set aside to cool.
Next, make the caramel. In a 2 quart saucepan, combine butter, brown sugar, corn syrup, and sweetened condensed milk and bring to a boil, stirring constantly. Continue stirring and allow mixture to boil for another 5 minutes. Remove caramel from heat and stir vigorously for a few minutes. Pour over shortbread and allow to set up at room temperature for at least 30 minutes.
For the chocolate, simply place chocolate in a microwave-safe bowl. Heat for 45 seconds, then stir and continue to heat and stir at 20 second intervals until chocolate is completely melted. Pour chocolate over the caramel layer and spread evenly to cover. Chill to set the chocolate - about 30 minutes.