Coated in buttermilk and fried until crisp, Southern-style fried chicken is the perfect partner in crime for bacon. A tangy spread of Dijon mustard-inflected mayonnaise keeps the richness of each bite in check and complements the flavors in the sandwich nicely.
Place all of the ingredients in a small bowl and whisk until thoroughly combined.
Place the chicken in a small nonreactive bowl and cover with the buttermilk. Cover with plastic wrap and refrigerate to marinate overnight or for at least 1 hour.
In a cast-iron skillet, cook the bacon until crisp, transferring to paper towels when done. Add the canola oil to the bacon drippings in the skillet and increase the heat to medium or 350 degrees, until the oil begins to bubble.
In a second bowl, combine the flour, cornstarch, salt, and pepper, mixing thoroughly. With kitchen tongs, remove the chicken breasts from the buttermilk and dredge in the dry ingredient mixture until the chicken is completely coated. Carefully place the chicken, using tongs, into the hot oil and brown until the breast is golden brown and crispy, for about 5 minutes per side.
Remove the chicken from the hot oil and drain on paper towels. Slice the rolls in half and lightly butter each side before placing butter side down in a sauté pan or skillet over medium heat until grilled.
For each sandwich: On the bottom roll, put 1 teaspoon of the dijonnaise, then layer on some lettuce. Place 1 more teaspoon of the dijonnaise on top of the lettuce and alternate the dijonnaise with the remaining ingredients in this order: tomato, chicken, and bacon. Cover with the top half of the hoagie roll. Repeat to make the other sandwich.