In a medium pot, heat chicken broth to a simmer.
5 cups chicken broth
Meanwhile, heat a dry skillet over medium-high heat and place the chiles in the skillet to toast, pushing down slightly and turning halfway through, about 30 seconds total. The chiles should slightly change color and have a toasty aroma. I toast my chiles one at a time to prevent burning and ensure each chile is properly toasted.
10 pasilla chiles
Remove stems, seeds, and veins from the chile peppers. Tear into large chunks and place in a glass bowl.
Once the broth is warm, pour enough broth over the chiles to cover them. Allow them to soak for about 7-10 minutes, or until softened.
Once the chile peppers are softened, place them in a blender with the soaking broth, onion, garlic, peppercorns, cumin, and oregano. Blend on high until smooth. This should yield about 3 cups of sauce.
1/4 white onion, 2 cloves garlic, 1 tbsp whole peppercorns, 1 tbsp whole cumin, 1.5 tbsp dried oregano
Heat the oil in a large skillet over med-high heat until shimmering (2-3 minutes). Strain the sauce into the skillet, using the back of a spoon to help push the sauce through the sieve. Add the remaining broth gradually to help push the pulp through the sieve.
2 tbsp vegetable oil
Place the pulp from the sieve back in the blender. Add a bit of sauce from the skillet to the blender and blend on high to process any remaining bits of chile and spices. Strain the remaining sauce into the skillet. Discard any remaining pulp.
Add the bay leaves to the sauce and salt to taste. Stir and heat to a simmer. Continue simmering about 8-10 minutes.
3 bay leaves, Salt to taste
Add in the shredded chicken, stir to incorporate. Taste the sauce for salt and adjust seasoning if necessary. Allow the chicken to heat through, about 5 minutes. Serve warm.
4 cups shredded chicken