Heat oven to 375℉/177℃.
Line sheet pans with parchment paper.
In a medium bowl, combine butter, both sugars, egg, egg yolk, and vanilla.
Whisk well.
Add flour, baking soda and chocolate chips.
Stir together.
Place half of dough in a small bowl.
Fold in melted chocolate. Set aside.
Fold peanut butter into remaining half of dough.
Use a 1½ Tablespoon scoop to portion peanut butter dough.
Use a 1½ Tablespoon scoop to portion chocolate dough.
Take one scoop of chocolate dough and break it into 4 or 5 pieces.
Press chocolate dough pieces around peanut butter dough.
Press together to get a slightly marbled effect.
Roll into ball.
Place on prepared pan and press down slightly.
Repeat with remaining dough.
Bake for 14 minutes, or until lightly brown around the edges.