Preheat the oven to 350°F (175°C) and lightly grease a mini muffin pan.
Beat together butter, peanut butter, brown sugar, and granulated sugar until well combined.
Add egg and vanilla extract, mixing until smooth.
Briefly mix in baking soda to distribute evenly.
Gently fold in all-purpose flour until just combined, resulting in soft, slightly sticky dough.
Roll dough into 1-tablespoon balls and place one in each cavity of the mini muffin pan.
Bake for 8 minutes, until edges are set but centers remain soft.
Immediately press one miniature Reese’s Peanut Butter Cup into the center of each hot cookie.
When the chocolate softens slightly, gently press a jumbo heart sprinkle onto each cookie cup.
Refrigerate the pan for 10–15 minutes to set the chocolate.
Carefully remove cookie cups from the pan and serve.