In a large bowl, use an electric hand mixer to cream together the butter, peanut butter, white sugar, and brown sugar until smooth. Beat in the egg and vanilla. In a separate bowl, combine the flour and salt. Beat the dry mixture into the creamed mixture just until slightly combined. Finish by mixing with a wooden spoon until crumbly and then use your hands to work it into a soft dough.
Preheat the oven to 350 degrees F.
Use a medium cookie scoop or spoon to scoop the dough out and roll 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. Use your fingers to reshape the cookie and smooth out any large cracks.
Bake for 11 to 12 minutes, until lightly golden brown on the bottoms. Cookies will seem soft but will set up as they cool. Immediately transfer the cookies to a wire rack to cool as soon as they come out of the oven. Allow to cool completely before filling with the ganache.
Chocolate Ganache Filling
Pour the heavy cream and corn syrup into a small heavy-bottomed saucepan and bring to a low simmer over MEDIUM heat. Remove the pan from the heat just before the cream begins to boil. Add the chocolate chips, but do not stir. Let the mixture stand, off the heat, for 5 minutes, and then stir until completely smooth. Allow the 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.