Add sweetened condensed milk and vanilla to chocolate peanut butter mixture and stir until well-combined. Pour into the prepared pan and smooth the top of the mixture with the back of a spoon.
Add chocolate, peanut butter, and butter to a microwave safe bowl. Heat between 1½ - 2 minutes on 80% power, depending on your microwave, until chocolate has begun to melt. Remove from microwave and stir until smooth.
Line an 8x8 baking pan with parchment paper, including an additional 2-inches of parchment paper to hang over both sides of the baking pan. Lightly spray the parchment paper with nonstick cooking spray. Set the baking pan aside.