In a medium saucepan, combine the sugar, butter, cocoa, milk, and vanilla. Bring to a slow boil and simmer until the sugar is completely dissolved. Remove from the heat.
Stir in the peanut butter and 2 1/2 cups oats. The mixture should be thick; stir in additional 1/2 cup oats if too runny. Let the mixture stand for about 10 minutes to cool.
Line a baking sheet with wax paper or parchment paper. Drop the mixture by heaping tablespoonful onto the lined sheet. Let the cookies stand at room temperature for 1 hour to firm up. Store in an airtight container.
Adapted from "The Comfort Table" by Katie Lee (C) Simon Spotlight Entertainment 2008. Provided courtesy of Katie Lee. All rights reserved.