Makes 18-20 cookies
1 cup all-purpose flour
Preheat the oven to 325 degrees Fahrenheit. Line two baking sheets with parchment paper or a silpat.
Sift together the flour, baking soda and salt in a medium bowl and set aside. In an electric mixer fitted with the paddle attachment, cream together on medium speed the butter, shortening, peanut butter and sugars until smooth and lighter in color, about 1 minute. Scrap the sides of the bowl with a rubber spatula. Add in the egg and vanilla extract and scrap again. Blend well. On low speed, gradually add the flour mixture until just combined. Using the spatula scrap the sides of the bowl again and fold in the peanut butter cups.
Drop heaping tablespoons of cookie dough onto the baking sheets, forming into balls, about 2" apart. Bake the two sheets in the oven for 15-20 minutes, rotating halfway through, or until the tops of the cookies are a very light golden brown. Cool the cookies on the baking sheets and then transfer to wire racks to cool completely.
Keep tightly covered at room temperature for up to 5 days.