chocolate swirl peanut butter fudge

I used to always make acorns (peanut butter balls dipped in chocolate) every year, but they are really a pain to make, even though I love them.  once I tried putting chocolate in my peanut butter fudge recipe, it worked out so well, I never made acorns again.

1 cup butter, plus more for greasing pan 

1 cup peanut butter 

1 teaspoon vanilla 

1 pound powdered sugar

½ cup mini chocolate chips

1. microwave butter and peanut butter for 2 minutes on high. stir and microwave on high for 2 more minutes.
   
2. add vanilla and powdered sugar to peanut butter mixture and stir to combine with a wooden spoon.
   
3. pour into a buttered 8 by 8-inch pan lined with waxed paper.  sprinkle mini chocolate chips on top (you can leave them on top or swirl them throughout the fudge).  place a second piece of waxed paper on the surface of the  fudge and refrigerate until cool.
   
4. cut into 1-inch pieces and store in an airtight container for up to a week.