Preheat oven to 300 degrees. Position shelf in center of oven. Cut a 15 inch square of heavy-duty aluminum foil. Invert the pan and center the foil over a 9-inch square baking pan, pressing it across the bottom and down the sides. Remove the foil, turn the pan right side up, and place the foil shell in the pan, shaping it smoothly across the bottom and snugly against the sides.
Strain the flour, rice flour, cocoa, salt, cayenne and cinnamon together 3 times. Set aside.
Place the butter in a medium bowl and mix on medium/low speed until smooth and creamy, about 1 minute. Add the sugar gradually, taking about 1 minute, then mix for 1 minute longer. Add the vanilla and mix to combine.
Transfer the mixture to a large, wide bowl for mixing. Using a wooden spoon, cut half of the dry ingredients into butter mixture until it is almost incorporated. Work in the remaining dry ingredients by hand, adding in five or six additions. Gently knead the mixture just until a smooth dough is formed. Do not overwork the dough.
Press the dough evenly into the pan. Be sure to push dough into corners.
Bake for 50 to 55 minutes, or until set on top. Take out and let rest for 5 minutes. Using dough scraper, cut straight down through the dough at about 1-inch intervals, making 8 strips. Give the pan a quarter-turn and cut 4 more strips at 2-inch intervals. Return to oven and bake for another 10 minutes. Remove from oven and let rest for 10 minutes.
Have a cookie sheet without sides ready. Using the foil as an aid, lift the shortbread from the pan and place it on the cookie sheet. Pull the aluminum foil so it releases from the sides of the shortbread. Wrap the loose sides of the foil around the cookie sheet. Cut through the shortbread again, and using a spatula, spread them slightly apart. Return to the oven for 10 more minutes to crisp the cookies. Remove from the oven and let cool 10 minutes before transferring to a cooling rack.
Melt the chocolate with 1/2 teaspoon of shortening. You may do this by warming in the microwave at 50% power for 30 second intervals for about 2 minutes. Take out and stir after each interval. When the nuggets have cooled, dip each end in the melted chocolate about 1/4 of the way up the cookie. Cool completely