Preheat oven to 375°F. Line two mini muffin tins with paper liners or grease them well with a flour-oil spray (like Bakers Joy). Set the tins aside.
Unwrap the miniature Reeses and/or Rolos if necessary and set the candies aside.
In a large mixing bowl, beat the ½ cup butter, ½ cup sugar, ½ cup brown sugar, and ½ cup creamy peanut butter together until light and fluffy (use an electric mixer or a wooden spoon).
Add the 1 egg and ½ teaspoon vanilla extract to the bowl and mix until combined.
In a medium bowl, whisk together the 1 ½ cups flour, ¾ teaspoon baking soda, and ½ teaspoon salt.
Stir the dry ingredients into the wet ingredients with a spoon until just incorporated; do not overmix.
Use a small cookie scoop to form 1-inch dough balls. Place one dough ball into each muffin well—do not flatten the balls.
Bake the cookie balls for 10–12 minutes, or until the tops are puffed and the edges are set.
Remove the tins from the oven and immediately press one miniature candy into the center of each warm cookie (if any candies are still wrapped, unwrap them before pressing). Press gently to create a cup shape without pushing the candy to the bottom.
If using Festive M & M candies, press one M & M onto the top of each chocolate piece before the chocolate sets.
Let the cookie cups cool completely in the pan so the chocolate firms up, then remove them from the tins and serve.