Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper or silicone mats. In a large mixing bowl, beat the softened butter and cream cheese together until smooth and creamy. Mix in the granulated sugar until fluffy. Beat in the egg and vanilla extract until fully incorporated. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined. Gently fold in the chopped Reese's peanut butter cups. Use a cookie scoop or tablespoon to drop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 12-15 minutes, or until the edges are set and the tops are slightly golden. Avoid overbaking for soft, cheesecake-like cookies. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Drizzle the cooled cookies with melted chocolate or peanut butter and sprinkle extra Reese's pieces on top if desired.