Cream the wet ingredients. In a large bowl, cream together the butter, peanut butter, granulated sugar, and brown sugar for 2–3 minutes until light and fluffy. Add the egg and vanilla extract and beat for another 2–3 minutes until smooth.
Mix in dry ingredients. In a separate bowl, whisk together the flour, oats, and baking soda. Gradually add the dry ingredients to the wet mixture and stir until just combined. Fold in the Grape Nuts cereal.
Scoop and bake. Using a cookie scoop or tablespoon, drop rounded dough balls onto a parchment-lined baking sheet. Bake for 9–10 minutes, or until the edges are lightly browned.
Cool and enjoy. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.