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1
Toast the guajillo and costeño amarillo chiles separately in a dry heavy pan over medium heat, pressing each flat for a few seconds per side, until the skin blisters and a faint smoke rises. Transfer each variety to its own bowl of hot water and soak until fully pliable. The chiles are ready when the flesh yields without resistance to a fingernail.
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2
Drain the soaked chiles, reserving a little soaking liquid. Blend the tomatillos raw with the drained chiles until the mixture is smooth. Pass the blended sauce through a medium sieve, pressing firmly on the solids, until the liquid runs through freely and only dry pulp remains.
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3
Heat the lard in a heavy cazuela or wide saucepan over medium-high heat until shimmering. Pour in the strained sauce all at once — it will spatter — and fry, stirring constantly, until the colour deepens from pale yellow-green to a deeper, more saturated ochre and the sauce pulls away from the sides of the pan. This step decides the dish: a sauce fried insufficiently will taste raw and flat; one fried correctly will carry the full earthy depth of amarillo.
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4
Dissolve the masa ball in a ladleful of warm chicken broth, working out all lumps, then stir the mixture into the sauce. Add the remaining broth in a steady stream, stirring to integrate. The sauce should coat a spoon in a light, even layer — thinner than negro, thicker than a plain chile broth.
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5
Reduce the heat to a low simmer — the surface should tremble, not boil. Submerge the raw chicken pieces in the sauce and braise gently, turning once, until the meat is cooked through and the juices run clear when the thickest piece is pierced. High heat at any point will break the sauce.
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6
Remove the pan from the heat. Lay the hierba santa leaves into the sauce and allow them to steep off the heat for two to three minutes only. The sauce is ready when a faint anise fragrance rises from the surface without any sharpness or bitterness — hierba santa added earlier or over heat turns acrid.