Prepare an 8-inch by 8-inch square baking pan or equivalent by spraying it with baking spray or cooking spray and lining it with parchment paper. Set aside.
Add the peanut butter and sweetened condensed milk to a medium pot over medium-low heat. Stir them together gently with a heat-safe silicone spatula, and let them heat up until very warm (not boiling).
Chop the white chocolate or break it into very small chunks (or use white couverture chocolate drops).
Take the peanut butter mixture off the heat and add the white chocolate, stirring the mixture very gently and slowly with a silicone spatula. The chocolate will melt slowly in the hot mixture.
Once all the chocolate pieces have disappeared and the mixture is smooth, scrape it into the prepared pan.
Press the fudge mixture down gently into the pan, smoothing out the top with the spatula.
Optional: If you want, top the fudge with some crushed peanuts, crushed M&Ms or crushed Reese's Pieces.
Refrigerate for about 2 hours to set (or overnight).
Slice into 16 small pieces and store in an airtight container in the fridge or freezer until ready to serve.