Preheat the oven to 350 degrees F. Spray a 12-cup muffin tin with cooking spray.
Beat the peanut butter, granulated sugar, light brown sugar and salt in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 5 minutes. Beat in the vanilla and egg on high until the dough comes together in a mass.
Pour some sanding sugar in a small bowl. Form the dough into 12 balls and roll to coat in the sugar. Put a dough ball in each muffin cup. Press the dough about 1/2 inch up the sides to make a shell. Fit a peanut butter cup into each dough shell. Bake until the edges of the cookies are set and golden, 10 to 12 minutes. Remove the tin to a cooling rack, then immediately top the peanut butter cups with a sprinkling of the peanuts. Cool 15 minutes, then remove the cookies to the wire rack to cool completely.
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