Preheat oven to 375 degrees. Unwrap the peanut butter cups and place in a bowl. Set aside.

Using a hand or stand mixer (as also be mixed manually), beat together the room temperature butter, granulated sugar and brown sugar. Mix until fluffy. Add in the peanut butter, egg and vanilla, and mix until well combined.

In a separate bowl, whisk together the flour, baking soda and salt. Gradually add the flour mixture to the wet ingredients a few spoonful's at a time. If using a mixer, stop to scrape down the sides of the bowl. Once all the dry ingredients are incorporated, the dough will be thick.

Using your hands, begin rolling the dough into 1" cookie dough balls. Place each dough ball into the mini muffin pan. Once the pan is filled, bake the cookies at 375 degrees for 9-11 minutes or until the cookies are puffy and slightly golden brown. Don't overbake.

Once the cookies come out of the oven, while still hot, press a single peanut butter cup into the center of each cookie. Once peanut butter cups have been places, using a table knife, gently run around the edge of the cookie, and lift it out. Cookies will come out easily.

Transfer baked cookies to a cooling rack to cool to room temperature.