In a large bowl, use an electric hand mixer to cream together butter, peanut butter, white sugar, and brown sugar until smooth. Beat in egg and vanilla. In a separate bowl, combine flour and salt. Beat into the creamed mixture just till slightly combined. Finish by mixing with a wooden spoon to form a soft dough. Cover and refrigerate dough for 30 to 60 minutes.
Preheat the oven to 350 degrees F.
Roll slightly chilled dough into 1 inch balls, and place balls 2 inches apart on parchment paper lined or ungreased cookie sheets. Make an indention in the center of each cookie using your thumb or index finger.
Bake for 12 minutes in preheated oven. Cool completely.
To make the chocolate ganache, pour heavy cream and corn syrup into a small heavy-bottomed saucepan and bring to a low simmer over medium heat. Remove pan from heat just before cream begins to boil and add chocolate chips but do not stir. Let stand, off the heat, for 5 minutes and then stir until completely smooth. Allow ganache to cool for 15 minutes. Fill each thumbprint with about 1 teaspoon of ganache.
To easily fill thumbprints, pour ganache into a plastic storage bag and snip the corner for easy dispensing. You can also use a plastic squeeze bottle to dispense or just use a small spoon.
Prep time does not include chilling time. Please allow for an additional 30 to 60 minutes.