In a large mixing bowl, whisk together the 2 cups suero de leche, 1 tbsp salsa picante como Frank's RedHot (if using), and 1 tsp sal kosher. Add the 3 lb piezas de pollo deshuesadas y con piel muslos, turning to coat thoroughly. Cover and refrigerate for at least 4 hours, or ideally overnight (up to 24 hours). The longer the soak, the more tender and flavorful the 3 lb piezas de pollo deshuesadas y con piel muslos.

In a large shallow baking dish or Ziploc bag, whisk together the 2 cups harina de trigo, ¼ cup almidón de maíz, 1 tsp sal kosher, 2 tsp pimienta negra, 2 tsp polvo de ajo, 2 tsp polvo de cebolla, 1 ½ tsp pimentón, 1 tsp pimienta de cayena, and 1 tsp tomillo seco until evenly combined.

Remove the 3 lb piezas de pollo deshuesadas y con piel muslos from the 2 cups suero de leche, letting any excess drip off but do NOT shake it dry — you want some moisture for the coating to cling to. Dredge each piece in the seasoned 2 cups harina de trigo, pressing firmly so the 2 cups harina de trigo adheres. Dip it back into the leftover 2 cups suero de leche briefly, then dredge in the 2 cups harina de trigo a second time, pressing firmly again.

Place the coated 3 lb piezas de pollo deshuesadas y con piel muslos on a wire rack set over a baking sheet. Let them rest uncovered for 15–20 minutes.

Pour the 4 cups aceite vegetal o aceite de cacahuete — into a large Dutch oven or heavy-bottomed pot until it is about 3 inches deep. Heat over medium-high heat until the 4 cups aceite vegetal o aceite de cacahuete — reaches 350°F on a meat thermometer.

Working in batches of 3–4 pieces (do not crowd the pot), carefully lower the 3 lb piezas de pollo deshuesadas y con piel muslos into the hot 4 cups aceite vegetal o aceite de cacahuete — using tongs. Fry dark meat pieces (thighs, drumsticks) for 13–15 minutes, and breast pieces for 12–14 minutes, turning occasionally, until deep golden brown. Between batches, let the 4 cups aceite vegetal o aceite de cacahuete — return to 350°F before adding the next batch.

The 3 lb piezas de pollo deshuesadas y con piel muslos is done when the crust is deep golden brown and an instant-read meat thermometer inserted into the thickest part (not touching bone) reads 165°F. Transfer to a clean wire rack set over a baking sheet — never to paper towels directly, as this traps steam and softens the crust. Let rest for 5 minutes before serving.