Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly.
In a large mixing bowl, cream together the softened unsalted butter, packed brown sugar, and granulated sugar until the mixture is light and fluffy. This should take about 2-3 minutes using an electric mixer.
Add the vanilla extract and the large egg to the creamed mixture, beating until well combined.
In a separate bowl, whisk together the cocoa powder, instant pudding mix, all-purpose flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined. Be careful not to overmix!
Stir in the brewed coffee (or water) until the dough is smooth.
Gently fold in the unwrapped mini Reese’s Peanut Butter Cups, ensuring they are evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a parchment-lined baking sheet, leaving some space between each cookie. Bake in the preheated oven for 10-12 minutes, or until the edges are set and the centers are soft.
Once baked, remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. Enjoy your Soft Chocolate Peanut Butter Cup Cookies warm or at room temperature!