Place oil in a medium-sized bowl and microwave for approximately 1 minute. In a bowl fitted for the mixer, add the oil, PBfit, vanilla extract, and powdered sugar, and mix until combined. Drizzle in the milk and mix till a soft dough forms. If the dough is still a little dry, add more milk.
Spread half of the fudge into a parchment-lined 8 by 8-inch pan, add half of the chopped peanut butter cups, and drizzle with peanut butter if desired. Top with remaining fudge and gently press down. Sprinkle on the remaining peanut butter cups and chopped peanuts and gently press down into the fudge. Chill for at least 2 hours. Drizzle with more peanut butter if desired. Let warm from the fridge for about 30 minutes before cutting into 1-inch cubes. Store in an airtight container in the refrigerator.