Melt suet in a large heavy skillet over medium-high heat. Remove and discard any solid pieces of suet, then add beef, in batches if necessary, and brown, turning occasionally, for 5 minutes.

Reduce heat to medium. Add garlic and chili powder, season to taste with salt and pepper, and mix well. Cook, stirring with a wooden spoon, for 1 minute, then sprinkle meat mixture with masa harina, and mix in thoroughly.

Gradually stir in stock, 4 cups warm water, and vinegar. Reduce heat to low and simmer, partially covered, stirring occasionally. Cook until meat is tender and begins to "melt" into sauce, about 4 hours; add water as necessary. Adjust seasoning with salt and cayenne.

Suggested garnish: Sprigs of cilantro