In a large mixing bowl, combine the peanut butter, softened butter, eggs, and vanilla (if using). Stir these ingredients together until they're well mixed.
Add the cake mix to the bowl about a quarter of the package at a time, stirring between each addition.
If it gets too hard to stir, you can use your hands to mix in the last bit of cake mix.
Just make sure all the dry ingredients are fully incorporated with the wet ones.
Pour some granulated sugar into a small bowl. This is for coating the cookies.