Preheat your oven to 425 degrees. Grease your skillet with 1-2 tablespoons of shortening, then preheat in oven for at least 5 minutes.
Combine all of your dry ingredients in a large mixing bowl and whisk thoroughly with a wire whisk. Add your shortening and cut in with your pastry cutter until it is broken down into pea-sized pieces.
Slowly add your milk, stirring until it is fully incorporated and a tacky dough forms. Do not overwork your dough.
Turn your dough out onto a floured surface and knead for one minute. Again, being careful not to overwork the dough.
Roll the dough into a ball, then flatten out with your hands. Using a rolling pin, roll it out into a 1/2 inch thick circle.
Carefully place the circle of dough into your skillet. Place into the oven and cook for 10 minutes, carefully flip the dough, then cook an additional 5 minutes. If you want a golden top to your pan de campo, turn your broiler on high for the last 3 minutes of cooking.