In a microwave-safe bowl, combine the peanut butter and butter. Microwave on high for about 2 minutes or until melted, stirring every 30 seconds.
Remove the bowl from the microwave and stir in the powdered sugar and vanilla extract until smooth and well combined.
Pour the mixture into a greased 9×9-inch baking dish and spread it evenly.
In a separate microwave-safe bowl, melt the chocolate chips in the microwave by microwaving on high for about 1 minute, stirring every 15 seconds until smooth.
Drizzle the melted chocolate over the peanut butter mixture in the baking dish. Use a knife or toothpick to create a marble effect by swirling the chocolate into the peanut butter mixture.
Refrigerate the dish for at least 2 hours or until the fudge is firm and set.
Once chilled, cut the fudge into squares and serve.
