01 -
Preheat the oven to 350°F (175°C). Line a muffin tin with paper liners to prevent sticking and facilitate 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 well blended. Gradually add all-purpose flour, mixing until a soft dough forms.
03 -
In a separate bowl, beat creamy peanut butter with powdered sugar until smooth. Shape this mixture into 24 to 30 small balls and refrigerate for approximately 10 minutes to chill.
04 -
Press a portion of cookie dough into each muffin cup, forming a shallow well in the center. Place one chilled peanut butter ball into each well. Top with semi-sweet chocolate chips and optional crushed add-ins. Add a small amount of remaining dough over the top to partially cover the filling.
05 -
Brush the tops of each assembled cookie bite with the egg yolk and water mixture to achieve a golden finish upon baking.
06 -
Bake in the preheated oven for 12 to 15 minutes, or until the edges turn lightly golden and the centers are just set.
07 -
Allow the cookie bites to cool in the muffin tin for about 5 minutes before transferring to a wire rack. The peanut butter center will be gooey when warm and firm up as they cool.