Combine beef, pork, mustard powder, white pepper, 2 tsp. kosher salt, pink salt, dextrose, and garlic powder in a bowl. Cover and let cure in the refrigerator overnight.
Transfer beef mixture to a plastic wrap-lined baking sheet and put into freezer until firm but not frozen, about 1 hour. Working in small batches and using a meat grinder (such as the grinder attachment on a Kitchen Aid stand mixer), grind meat through medium die, alternating pieces of meat and fat. Return ground meat to freezer until firm but not frozen, about 45 minutes more. Working in small batches, grind meat and ice through small die.
Put ground meat and powdered milk into bowl of a stand mixer. Using the paddle attachment, mix on low speed, stopping to clear paddle as needed, until smooth, about 2 minutes. (To check for seasoning, pinch off 1 tsp. of the meat mixture and cook in a small skillet. Season meat mixture with more salt, if you like.) Tie one end of pork casings with kitchen twine. Using sausage-stuffing attachment, stuff sausage into pork casings. Tie open end of sausage with kitchen twine and twist sausage into 6″ links; transfer to a rack set in a baking sheet. Refrigerate overnight.
Build a low fire in a charcoal grill or smoker and add hickory, oak, or cherry wood chips. When temperature drops to 250° add sausages and smoke them, adding more coals as needed to maintain a temperature of 200° and flipping sausages occasionally, until they are red and firm throughout, about 2 hours. Add more hot coals and grill sausages over medium-hot fire until slightly charred; serve immediately with mustard. Alternatively, let smoked sausages come to room temperature, chill for up to 3 days, and grill or pan-fry before serving.