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.
Add chocolate, peanut butter, and butter to a microwave safe bowl. Heat between 1 1/2 - 2 minutes on 80% power, depending on your microwave, until chocolate has begun to melt. Remove from microwave and stir until smooth.
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.
Allow the fudge to sit at room temperature for about 3-5 minutes to begin to firm slightly. Sprinkle with sea salt.
Place baking pan into the refrigerator to chill until hardened, about 2 hours or the freezer for about 30 minutes.
To cut the fudge, lift the fudge out of the baking pan using the overhanging parchment edges. Cut into 1-inch squares.
Store fudge in an airtight container lined with parchment paper in the refrigerator.
Notes
Make Ahead - this recipe makes a wonderful make ahead candy recipe. While it is quick and easy to make right before serving, you can make ahead to make sharing even easier. Store - If storing cut into pieces, arrange in an airtight container in layers with parchment or waxed paper to divide the candies. You can store in an airtight container in a cool place for up to a week or in the refrigerator for up to 3 weeks. Freeze - Wrap the uncut block of fudge well with plastic wrap and then foil, store in the freezer up to 3 months. Place pieces of waxed paper between layers of cut fudge in an airtight, freezer-safe container and store for up to 3 months.