In a medium bowl, mix the peanut butter, butter, brown sugar, and powdered sugar together.
Melt the chocolate chips in a microwave-safe bowl for 2 minutes on 50% power (power level 5). Stir until bowl no longer feels warm, then microwave for 30 seconds more. Stir until smooth.
Place enough melted chocolate into the heart mold to cover the bottom of each heart. Gently place about 1-2 teaspoons of the peanut butter mixture into each heart. (It helps if you can shape it into a rough disc before placing it into the mold.) Cover with more of the melted chocolate. Place the hearts into the refrigerator or freezer until hardened.
Store in an airtight container in the refrigerator.
If you have extra peanut butter filling and chocolate, you can layer the chocolate and peanut butter into the bottom of a muffin liner to make your own homemade peanut butter cups! Or make peanut butter balls. Yum!