In a 6" pan, combine melted butter and flour. Stir to combine until a smooth paste is formed. Add in the 6 ounces of evaporated milk and stir. Cover the pan with foil or a silicone cover.
Add 2 cups of water to the inner pot of the Ninja Foodi. Add in 8 ounces of elbow macaroni and stir so most of the pasta is under the water.
Place the rack on top of the macaroni in the high position. Place the pan with the roux on top (covered). Put on the pressure lid and turn the valve to seal. Set the pressure to high for 3 minutes. When the time is up, allow the Ninja Foodi to naturally release the pressure for 2 minutes, then manually release the remaining pressure.
Grate the 8 ounces of cheese, you should have about 2 cups of grated cheese. *You can use any types of cheeses that you like.
Remove the pan and the rack. Stir the roux mixture and pour into macaroni and stir. Add grated cheese and stir to melt.