The following recipe is an exciting foray into Tex-Mex/Asian fusion cuisine: Char sui and a Tex-Mex marinade.
Throw the pork butt, garlic and chili powder into a zip-lock bag. Cover the meat with white vinegar. Massage the marinade in well. Seal and store the bag in the refrigerator for at least 6 hours.
Preheat the oven to 240-250 degrees F. Place an oven rack on the top-most rung. Put a metal pan, half-full of hot water on the floor of the oven.
Shake off the excess marinade from the pork. Drive a metal (shish kebob) skewer through the middle portion of the meat, width-wise. Bend 2 trussing needles into "S" hooks. Hook each side of the skewer with the lower side of each "S" hook. Hang the pork on the grate of the oven with the top-sides of the hooks, directly over the water pan.
(Keep an eye on the water in the pan. It will evaporate. Fill it with more hot water, when needed.)
Slowly roast the pork until finished (5-6 hours). Carefully brush with the excess marinade every hour. After removing the pork from the oven, season it with salt. Allow to rest for 10 minutes. While the butt is resting, make the sauce.
Melt the butter in a sauce pan. Sprinkle in the flour and wisk constantly until the roux becomes light brown. Break down the roux with milk by incorporating it slowly. Season to taste with salt. Allow the sauce to reduce until very thick and bubby. Throw in the cheese. When the cheeses melt, turn off the heat.
De-seed and slice the chipotles into thin strips. Slice the pork into long strips. Sprinkle the pork with chili powder. Wrap the meat and chipotle strips up into pre-warmed tortillas. Serve the cheese sauce on the side (or in small bowls) for dipping.