Line an 8x8-inch square baking pan with parchment paper, or foil. Set aside.
Add the butter to a microwave-safe bowl and heat until melted. Stir in the sugar and vanilla, then fold in the oats until completely coated. Microwave for 1 minute, then stir.
Press half of the mixture into the bottom of the prepared pan.
In a microwave-safe bowl, add the chocolate and peanut butter. Heat in 30 second intervals, stirring after each one, until melted and smooth. Fold in the chopped peanuts.
Pour the chocolate over the oats mixture, and spread out evenly. Top with the remaining oats mixture, and press down gently.
Cover and refrigerate for a least 3 - 4 hours, or overnight before slicing into bars.
Video
Notes
These bars can be stored in an airtight container at room temperature or in the fridge for up to 2 weeks. They also freeze well for up to 3 months. Thaw overnight before serving.