Combine the buttermilk, 1 tbsp salt, and the red-pepper sauce, if using, in a large airtight container. Add the chicken pieces, turning to coat in the liquid. Cover and refrigerate at least 2 hours or overnight
Combine the flour, the remaining 1/4 cup salt, the black and cayenne peppers, and the baking powder in a large brown paper bag. Shake vigorously
One at a time, place the chicken pieces in the bag, and shake to coat. Place the coated pieces on a clean plate or tray. Heat the vegetable shortening (and bacon drippings, if using) in two 10-inch cast-iron skillet over medium-low heat (one large cast-iron skillet may be used; it will just take longer to fry all the chicken).
Using a frying thermometer to measure the temperature, bring the shortening to 375 F; it should be at medium, not a rolling boil. Use tongs to place the thighs and drumsticks in the skillets. Fry until the coating is dark golden on the bottom, 10 to 14 minutes, then using tongs, turn the chicken over. Cook until the coating is dark golden, another 10-14 minutes. An instant-read thermometer inserted into a thigh should register 190 F
Drain on absorbent layers of paper towels. Using slotted spoon, remove any bits of crisp coating left in the skillet, and discard. You don't want this to burn when you fry the next batch
Place the breasts and wings in the skillets. Cook 10-14 minutes on each side. Drain on absorbent layers of paper towels. Serve warm
Notes
TIPS FROM MARTHA STEWART ON FRIED CHICKEN 101: 1. Use baking powder in the batter causes the batter to puff up in the hot oil, giving it extra crunchiness 2. Marinate chicken overnight in buttermilk gives it a rich, slightly tangy flavor 3. Fry the chicken in shortening with a bit of bacon fat to produce chicken that is crispy, juicy, and light, with just a hint of the bacon's smokiness 4. Use a cast-iron skillet to allow the chicken to brown evenly without burning because it keeps the fat hot and at an even temperature