Preheat the oven to 325 °F. Line a half sheet pan with parchment paper or a silicone baking mat.
In the bowl of a stand mixer or in a mixing bowl with a hand mixer, cream16 oz unsalted butter and 8 oz granulated sugar until light and fluffy. Add 20 oz all purpose flour and 1/4 teaspoon salt all at once and mix just until combined.
Dump the dough onto a lightly floured surface and form the dough into rectangle shape. Transfer the dough to the prepared sheet pan and use your hands to work the dough to fill the pan.
Lay another sheet of parchment over the dough and rub over the surface to even out the thickness. Remove the parchment and use a fork to prick the top of the dough all over. Bake until the dough is just beginning to brown and the center of the dough is set, about 20 minutes.
Cool for 5 minutes then sprinkle the chopped 12 oz dark chocolate over the warm shortbread. Allow the chocolate to melt without touching.
Once the chocolate is completely melted use a small spatula to spread the chocolate to cover the shortbread in an even layer. Set it aside until the chocolate is completely set
Melt 12 oz white chocolate in a microwave in 30 second increments, stirring in between increments (be careful, white chocolate burns easily you must stir often)
Pour the melted white chocolate over the dark chocolate and smooth into an even layer. Before the white chocolate sets, sprinkle the candy cane bits over the white chocolate and gently press the candy bits into the surface of the chocolate so they stick. Allow to set completely before cutting.
To cut use a long serrated knife. Run the knife under hot water (or dip in a pitcher of hot water) then wipe the blade clean.
Use a gentle sawing motion to cut through the candy bits and chocolate layers before cutting into the shortbread. First trim the ragged edges, then cut the sheet into 48 squares. Repeat warming and wiping the knife between each cut.