Line a 9x9-inch baking dish with parchment paper, leaving some overhang for easy removal later.
In a microwave-safe bowl, combine the semi-sweet chocolate chips and 1/2 cup of peanut butter. Microwave in 30-second intervals, stirring in between, until melted and smooth. This should take about 1-2 minutes total.
Pour the melted chocolate mixture into the prepared baking dish and spread it evenly with a spatula.
In another bowl, mix the remaining 1/2 cup of peanut butter, powdered sugar, and vanilla extract until well combined and smooth.
Spoon dollops of the peanut butter mixture over the chocolate layer. Use a knife or spatula to swirl the two mixtures together for a marbled effect.
If desired, sprinkle crushed peanuts and mini marshmallows on top for added texture and flavor.
Refrigerate the bark for at least 2 hours or until fully set.
Once set, lift the bark out of the baking dish using the parchment paper overhang. Cut into squares or break into pieces.