In a 3-quart saucepan or 12-inch high-sided skillet, melt the butter over medium heat. In a small bowl, whisk together the flour, kosher salt, garlic powder, dry mustard, and cayenne pepper. Sprinkle the seasoned flour over the melted butter and whisk for about 30 seconds to form a loose roux.

Moving very slowly at first and whisking constantly, pour the water, whole milk, and evaporated milk into the pan in a thin stream. Stop frequently to whisk completely smooth before adding more liquid to prevent lumps from forming.

Add the dry elbow macaroni directly to the pot. Raise heat to medium-high and bring to a simmer, stirring every minute to prevent sticking. Once slow bubbles appear from the center of the pan, reduce heat to medium-low. Cook and stir every 1 to 2 minutes for about 10 minutes until pasta is al dente. Do not drain.

Reduce heat to low. Add the cheese in three additions, starting with the American cheese or Velveeta, then adding the cheddar in two batches. Stir each addition fully until melted and smooth before adding the next to prevent a grainy sauce.

Serve immediately. If the sauce tightens up before serving, stir in up to 1/2 cup of whole milk over low heat until the sauce loosens to your preferred consistency.