Grease a 10x13 pan (or 8x8 for thicker fudge).
Stir sweetened condensed milk and confectioners sugar in a microwave-safe bowl until combined.
Microwave on high for 3 minutes.
Immediately add peanut butter and vanilla. Stir vigorously until smooth.
Pour into the prepared pan and smooth the top.
Allow to cool completely at room temperature, then chill in the fridge before cutting.
For Chocolate Layer: Stir 4-5 tbsp cocoa powder into the milk/sugar mix before microwaving, and reduce peanut butter to 1/4 cup.