01 -
Preheat oven to 350°F (175°C). Line a muffin tin with paper liners to prevent sticking and ease removal.
02 -
In a large bowl, cream together softened butter, brown sugar, and granulated sugar until light and fluffy. Incorporate the egg (if using) and vanilla extract until fully combined. Gradually add all-purpose flour, mixing until a soft dough forms.
03 -
In a separate bowl, beat peanut butter and powdered sugar until smooth. Form the mixture into 24 to 30 small balls and refrigerate for about 10 minutes to firm up.
04 -
Press a portion of dough into each muffin cup, creating a well in the center. Place a chilled peanut butter ball into each well. Top with semi-sweet chocolate chips and optional add-ins. Cover partially with a small amount of remaining dough.
05 -
Brush the tops of each assembled bite with the egg yolk and water mixture to achieve a golden finish.
06 -
Bake for 12 to 15 minutes, until edges are lightly golden and centers are just set.
07 -
Allow the cookie bites to cool in the muffin tin for 5 minutes before transferring to a wire rack. The peanut butter center will be gooey when warm and will set as it cools.