Lightly grease an 8 x 8-inch baking pan. Line the pan with parchment paper, allowing the excess paper to overhang the sides. Grease the paper and any exposed pan with butter and set aside.
In a large, heatproof bowl combine the sweetened condensed milk, white chocolate, peanut butter chips, and butter; set aside. Fill a saucepan with a little water (you want it about 1/4 full) and bring it to a boil. Reduce the heat to barely a simmer. Place your heatproof glass bowl holding your ingredients on top of the pot. Stir frequently, until the white chocolate and peanut butter chips have melted and the mixture is completely smooth, about 10 - 12 minutes. Remove from heat. The mixture will be very thick! Using a sturdy spatula, scrape the fudge into the prepared pan and smooth the top. Sprinkle with sea salt and press on a few mini peanut butter cups, if using. Refrigerate for at least 2 hours.
Allow the fudge to rest at room temperature for at least 10 minutes before cutting. Then gently run a butter knife around the edge of the pan to loosen the fudge. Remove the fudge and transfer it to a cutting board. Peel off parchment paper, then slice the fudge into small pieces. Serve at once or store in the refrigerator for up to 1 month. You may also freeze the fudge, in an airtight freezer safe container for up to two months.
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Recipe by Baker by Nature at https://bakerbynature.com/easy-peanut-butter-fudge/