Combine peanut butter, milk, brown sugar, butter, cocoa and salt in a medium saucepan. Cook over medium heat, stirring, until the butter and sugar have melted. Remove from heat and stir in oats and vanilla.
Drop the dough by generous tablespoonfuls onto a parchment-lined baking sheet. Press into 2-inch circles. Chill until firm, at least 30 minutes. Keep refrigerated.
Equipment: Parchment paper
To make ahead: Cover and refrigerate for up to 5 days.