Preheat oven to 350 degrees.
In a large bowl, cream the sugar, brown sugar, and butter together.
Add the vanilla, eggs, and 1 cup peanut butter and stir to combine.
Then add oatmeal, flour, baking soda, and salt and mix together.
Roll dough into 1" balls and place on an ungreased cookie sheet.
Bake for 10-12 minutes, or until golden brown.
Melt remaining peanut butter in the microwave for about 30 seconds and stir until smooth.
Spread a spoonful of melted peanut butter on top of each cookie.
Let cool until the peanut butter has become solid again (I usually throw them in the fridge so they set up faster and so I can eat them sooner).
While cooling, mix together all of the frosting ingredients with a hand mixer and then spread on top of cookies.