Peanut butter needs to be store-bought, not natural. Recommended brands are Skippy or Jif.
Condensed milk can be substituted with more corn syrup. If no condensed milk is used, leave out the extra milk.
The sauce hardens when cooled. To make runny again, microwave for 20-30 seconds, then mix well. Add 1-2 teaspoons of milk if needed.