Cream the butter and sugar. Cream butter and sugar together until light and fluffy, about 2 minutes.
Mix in the wet ingredients. Then mix in the egg, egg yolk, and vanilla extract.
Mix in the dry ingredients. Mix in the flour, cocoa, salt, and baking powder until just combined.
Scoop the cookie dough. Scoop the cookie dough into one tablespoon-sized portions, then roll them between your hands to make them round. Place them on a parchment-lined baking tray with some room in between each.
Indent the centers of the cookies. Indent the centers with the round back of a teaspoon or by sticking your thumb straight down into the center. Chill the tray of thumbprint cookies in the freezer for 2-3 hours, or until frozen solid. When ready to bake, preheat the oven to 350 F/180 C.
Bake the cookies. Bake the frozen chocolate thumbprints spaced 1 ½-2 inches apart for 7-9 minutes. Right out of the oven, use a small round cookie cutter to scoot the edges of the cookies to make them perfectly round. Then reindent the centers (they will puff up some while baking). Let the tray cool on a wire cooling rack while you make the ganache.
Make the chocolate ganache. Heat the cream and espresso (if using) in a pot or the microwave then pour it over the chocolate chips. Let them sit for 1-2 minutes, then stir to emulsify the ganache. It should be smooth and silky with no chocolate chunks!
Fill the thumbprints. Fill the center of the cookies with the chocolate ganache. Sprinkle finely chopped chocolate on the edges if desired and enjoy!
Store the baked cookies in an airtight container or ziplock bag for 2-3 days at room temperature. You can store the frozen cookie dough balls in an airtight container or freezer bag for up to 3 months.