Cream together softened butter and sugars. Add peanut butter and vanilla extract, mix. Add egg, mix.
Sprinkle flour, oats, baking soda, and salt over the mixture and combine. The dough will be soft and just a bit sticky.
Use a 1/3 cup scoop or measuring cup to scoop/measure 7-8 cookies. Lightly pat down the tops so they are slightly flattened. Bake for about 16-18 minutes.
Once cookies are completely cool, frost with chocolate fudge buttercream.
Cream together butter and confectioners' sugar until fluffy. Add melted & cooled chocolate, cream, and vanilla extract. Beat until fluffy.
Spread frosting onto the cooled peanut butter oatmeal cookies and store in the fridge.