Shortbread: Preheat oven to 300°F. Line a baking sheet with parchment. In the bowl of an electric mixer fitted with the paddle attachment, beat together flour, salt, sugar and butter on medium-low speed until a smooth dough forms, about 5-7 minutes. This dough depends a lot on the moisture content of the butter. If your butter is particularly dry, add more butter 1 tablespoon at a time until the dough comes together. Chill in the refrigerator until dough is firm, about 15 minutes. On a floured surface, roll dough to ¼ inch thickness. Using a 2-½ inch square cookie cutter, cut out squares and place on prepared baking sheet. Bake rotating halfway through until shortbread is lightly golden, about 30-35 minutes. Chocolate Frosting: Melt chocolate in a double boiler; stir occasionally until smooth. In a separate bowl, stir together the cocoa and boiling water. Let both the chocolate and the cocoa mixture cool to room temperature. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, sugar, and salt on medium-high speed until light and fluffy. Add the chocolate on low speed, and then beat in the cocoa mixture. To Assemble: Spread or pipe frosting on each cookie and top with a Ghirardelli Chocolate Square.