Add water and sugar to small saucepan on medium heat. Stir until sugar fully dissolves and mixture begins to boil.

Dissolve cornstarch in a small amount of water (about a tablespoon). Add to saucepan. Stir while mixture boils for a minute or two. Sauce should start to appear viscous and may be cloudy.

Add peanut butter to saucepan. Stir while peanut butter begins to melt and combine. Once mixture begins to boil, it thickens quickly. Remove from heat.

Stir in airtight container (like a mason jar) for up to a week on the counter.