Line an 8x8 square pan with parchment paper or aluminum foil.
In a medium-sized microwave-safe bowl, melt chocolate chips and butter on high 90 seconds, stirring once halfway through. If the chips are still not completely melted, continue to microwave in 10-second increments, stirring each time, until smooth.
Remove 2 tablespoon of sweetened condensed milk from the can and set aside. Pour the remaining sweetened condensed milk into the melted chocolate, along with 1 teaspoon vanilla extract, and peanut butter whiskey. Stir until combined. Pour into the 8x8 pan and spread evenly.
Place the peanut butter, 2 tablespoons butter, 1 teaspoon vanilla extract, and the reserved 2 tablespoons sweetened condensed milk in a separate bowl. Microwave in 30-second increments until the butter is melted. Mix well.
Drizzle the peanut butter over the fudge layer. Use a knife to cut through the peanut butter in a zigzag pattern to make a swirl.
Refrigerate for 2 hours, then slice into squares.
Storage: Wrap your fudge with wax paper and store in an airtight container. It will keep in the refrigerator for up to 3 weeks.Substitutions:
If you do not have peanut butter whiskey than regular whiskey can be used.
Milk or dark chocolate can be substituted for semi-sweet.