Preheat oven to 350°F. Lightly grease a 24-count mini muffin pan. Freeze the Reese’s cups while preparing the dough.
In a large bowl, beat together the butter, peanut butter, brown sugar, and granulated sugar for 2–3 minutes until light and fluffy.
Mix in the egg and vanilla extract until smooth and fully combined.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt.
Gradually mix the dry ingredients into the wet ingredients just until combined. Do not overmix.
Chill the dough for 30 minutes.
Scoop approximately 1 tablespoon of dough into each muffin cavity, filling each about 3/4 full.
Bake for 9–10 minutes, until the edges are lightly golden and the centers still appear slightly soft.
Immediately press one frozen Reese’s cup gently into the center of each cookie cup.
Cool in the pan for 15 minutes before removing to a cooling rack.
Top with flaky sea salt or melted chocolate drizzle if desired.