Cream the butters and sugars. Add the vanilla and egg. Stir in the flour, baking powder, soda, and salt till well combined; stir in the oatmeal.
Drop by teaspoonfuls onto lightly greased cookie sheets. Press each cookie twice with a wet fork (In a crisscross pattern) to make 1/4 inch thick cookies. I use a small bowl of water to keep the fork wet.
Bake at 350 degrees F for 10 minutes or until light brown. Cool on wire racks.
To make filling: Cream all ingredients until smooth. Spread filling between two cookies to form sandwiches and hide from your husband.