1. Preheat your oven to 325°F for a softer texture or up to 375°F for a crisper outer shell, and gather all your ingredients, ensuring the butter is softened and egg yolks are at room temperature.
2. In a separate bowl, mix the dry ingredients: all-purpose flour, Dutch process cocoa powder, salt, and baking powder. Ensure they blend evenly.
3. In a large mixing bowl, cream the softened unsalted butter with brown sugar and granulated white sugar until the mixture becomes light and fluffy.
4. Add the egg yolks and vanilla extract or paste, mixing thoroughly after each addition.
5. Gradually incorporate the dry ingredient mixture into the wet ingredients, stirring until just combined, resulting in smooth and pliable dough.
6. Scoop uniform balls of cookie dough and place them on a prepared baking sheet. Create an indentation in the center of each dough ball using your thumb or the back of a spoon.
7. Bake the cookies in the preheated oven for 9-12 minutes, depending on your preferred texture. Immediately repress the indentations gently after baking, then allow the cookies to cool completely.
8. While the cookies cool, prepare the ganache by melting semi-sweet or bittersweet chocolate chips with heavy cream over a double boiler or in the microwave, stirring until smooth. Add optional flavorings such as vanilla or a splash of bourbon if desired.
9. Fill the cooled cookies’ thumbprints with the slightly cooled ganache.
10. Optionally sprinkle the cookies with powdered sugar, decorative sprinkles, or sea salt while the ganache is still soft.
11. Let the ganache set at room temperature before serving your rich and chewy Chocolate Thumbprint Cookies.