Easy Chocolate Peanut Butter Fudge

1 regular-sized package of milk chocolate chips
1 can sweetened condensed milk
1/2 cup chopped walnuts (optional)
1 tsp. vanilla
2/3 cup peanut butter

Mix all ingredients together in a microwavable bowl. Heat in microwave for 3 minutes on high. Stir and heat again for an additional 2 minutes. Pour into a 8x8 or 9x9 pan and refrigerate for a few hours before cutting.