In a microwave-safe bowl, stir together water and Alka-Seltzer. Let stand until fizzing stops, about 1 minute.
Add butter to the bowl and microwave until the butter is melted, about 30 seconds.
Add the shredded cheese to the bowl. Microwave in 30-second increments until cheese is melted and bubbly.
Using a fork or small whisk, stir until the sauce is smooth, adding any optional flavorings if desired. Let stand until perfectly thickened, 1 to 2 minutes.
