• Preheat your oven to 350 degrees F and line two baking sheets with parchment paper.
• In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder. Set aside.
• In a large bowl, cream together the butter, brown sugar, and white sugar until fluffy. Add the egg yolks and vanilla, mixing until pale and fluffy.
• Gradually add the dry ingredients to the wet ingredients, mixing until combined.
• Scoop the dough into 34 portions and roll them into balls. Make an indent in each ball using a ¼ teaspoon.
• Chill the dough balls in the fridge for at least one hour.
• Bake the chilled dough balls for 9-11 minutes, then reshape the indents with a cookie cutter while they're still warm.
• Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a cooling rack to cool completely.

**For the Chocolate Ganache:**

• Place the chocolate chips in a small bowl.
• Heat the heavy cream until almost boiling, then pour it over the chocolate chips. Let it sit for 1 minute.
• Stir until the chocolate is completely melted and smooth.

**Assembling the Cookies:**

• Fill each cookie well with about 1 teaspoon of ganache, then sprinkle with optional sprinkles.
• Chill the cookies in the fridge for 10-15 minutes to allow the ganache to set.
• Store any leftovers in an airtight container for up to three days.
• Now, grab a glass of milk and enjoy these decadent Chocolate Thumbprint Cookies!