Preheat your oven to 350 F (175C) and line your baking sheets with parchment or a silicone baking mat.
In a large mixing bowl, use an electric mixer gradually turned up to medium-high to beat together the butter and shortening until light and fluffy. Add the sugar, brown sugar, sour cream, and vanilla, and beat again until well combined. Finally, beat in the egg until blended.
In a separate bowl, add the flour, baking soda and salt. Gently whisk these together to incorporate all the ingredients. With your mixer on its lowest setting, slowly add in the dry ingredients mix until everything comes together.
Finally stir in your mini peanut butter cups. At this point, I recommend chilling the dough for about 30 minutes.
When you're ready to bake your cookies, scoop out about a quarter cup of dough for each cookie onto your baking sheet. Make sure you leave plenty of space for them to grow. I have small baking sheets, so I can only fit three or four at a time.
Bake your cookies for about 14 minutes or until golden around the edges. Allow them to cool on the baking sheet for about five minutes before transferring them to a wire rack to cool completely. Store in an air tight container.