PREP: Spray a large (6 quart) slow cooker with cooking spray. Pour the chicken broth in the bottom of the slow cooker. Trim off fatty parts of chicken thighs (some fat is fine; most will render off, I just slice off the big pieces on the sides). Combine the brown sugar, chili powder, paprika, cumin, and garlic powder in a small bowl. Add salt and pepper to personal preference; I add 1/2 tablespoon fine sea salt and 1/2 teaspoon pepper. Stir and then rub evenly over the chicken.
COOK CHICKEN: Place the chicken and chicken broth in the slow cooker in an even layer. No need to stir. Cover and cook on low for 4-6 hours; it takes about 4 and 1/2 to 5 hours in my cooker.
SHRED CHICKEN: Once the meat is completely cooked, remove the chicken to a large bowl. Shred with two forks.
SAUCE: When the chicken is almost finished cooking, prepare the sauce. For the sauce, combine all of the ingredients in a blender or food processor. Season to taste with salt and pepper, I add 1/4 teaspoon of each. Blend or process until completely smooth. Taste and adjust ingredients to preference. You may want more chipotle for a hotter sauce (but add slowly!) or a little more salt or pepper. Toss about half of the sauce with the shredded chicken to warm it through. Add sauce gradually just until the chicken is nicely coated. Taste and adjust salt/pepper.
QUICK "PICKLED" RED ONION: (See Note 1)
CHAR TORTILLAS: Spray both sides of the tortillas with olive oil cooking spray and char the tortillas directly over the gas flames for a few seconds. Use tongs to flip until tortillas are lightly charred and soft. Immediately fold the tortilla in half (so it will fold into a taco nicely later) and set aside.
ASSEMBLE: Top charred tortillas with saucy chipotle chicken. Add your favorite toppings; we like drained, rinsed, and warmed black beans, thinly sliced ripe avocado, pickled red onions, a sprinkle of queso cotija cheese, fresh cilantro, and a few squeezes of fresh lime. If desired, add additional chipotle sauce to tacos or dip the tacos in the sauce. Any leftover sauce will stay good for 5-7 days in an airtight container in the fridge and can be tossed with cooked chicken or pork. Enjoy while hot!