Boil the elbow macaroni noodles in a large pot of salted water over medium-high heat according to the package instructions until al dente.
In a separate medium saucepan melt the 2 ounces of cream cheese and quarter cup of sour cream together over medium-low heat. Add in ground mustard powder, and ¼ cup of pasta water when the noodles are almost done boiling.
Stir sauce together and add handfuls of shredded cheese. Repeat, adding more cheese as the cheese melts into the sauce.
Drain the macaroni noodles from the water and place in a large bowl. Pour cheese sauce over the noodles and gently combine everything with a wooden spoon.
Serve immediately.
Notes
If your pasta water isn't ready yet you can start melting cheese into the sauce and adding the water later. You want to use the pasta water when the noodles are almost done boiling because it will have more starch and binding power.