Place the sweetbreads in a sealable container, cover with cold water, and soak for at least 2 hours.
Discard the soaking water and place the sweetbreads into a pot large enough to hold them. Cover with cold water and bring to a boil. Allow the sweetbreads to cook until they are white and puffy, about 5 minutes. Drain sweetbreads in a colander and then place in a bowl with ice water to stop them from cooking.
Take the cooked sweetbreads and pat dry with a paper towel. You’ll see a thin membrane, which you need to peel off of the meat. Chop off any gristly bits and then chop the sweetbreads into 1-inch sized pieces.
In a sealable container, place the chopped and cleaned sweetbreads and cover with the buttermilk, jalapeño, garlic and cumin. Soak for at least 2 hours. Drain and rinse the sweetbreads. Mix together the flour, salt, black pepper and cayenne. Taste and adjust seasonings. Lightly sprinkle the sweetbreads with salt and pepper and then dredge into the flour.
In a large skillet on medium low, heat up the butter. Add the floured sweetbreads and cook on each side until lightly browned and crisp, about 5 minutes. Serve with warm tortillas, pico de gallo and sour cream.
Notes
Sweetbreads are highly perishable and need to be cooked within a day of purchasing. Once cooked, however, they can keep in the refrigerator for a couple of days.