Line a cookie sheet with parchment paper or the Silicone Baking Mats to prevent the cookies from sticking.
In a large heavy saucepan over low heat, add sugar, butter, milk or water, and cocoa powder; stir until melted. Increase heat to medium-high and bring just to a boil, stirring constantly so the mixture does not burn. Remove from heat.
Add peanut butter, oats, and vanilla; stir until well mixed.
While mixture is still warm, drop dough by rounded tablespoonfuls onto the prepared baking pans. Let sit until cool and the chocolate is hardened.
Mountain Bar No-Bake Cookies Recipe: https://whatscookingamerica.net/Cookie/MountainBarCookies.htm