To Make the Cookies:

1. Prepare Baking Sheets: Line two baking sheets with parchment paper and set aside.
2. Mix Dry Ingredients: In a medium bowl, whisk together flour, cocoa powder, salt, and baking powder. Set aside.
3. Cream Butter and Sugar: In a large bowl, cream together softened butter, brown sugar, and granulated sugar using an electric mixer for about 2 minutes until fluffy.
4. Incorporate Wet Ingredients: Mix in the egg yolks and vanilla extract until the mixture becomes pale and fluffy.
5. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients and mix until just combined.
6. Shape Dough: Scoop the dough into 34 portions (about 1 tablespoon each) and roll into balls. Place the dough balls on the prepared baking sheets.
7. Create Indents: Using the back of a ¼ teaspoon, press down on each dough ball to create an indent.
8. Chill Dough: Refrigerate the dough balls for at least 1 hour.
9. Bake: Preheat the oven to 350°F. Bake the cookies for 9-11 minutes.
10. Press Centers: While the cookies are still hot, gently press down on the indents again using a ¼ teaspoon. Allow cookies to cool completely on the baking sheet before transferring to a wire rack.

To Make the Ganache:

1. Prepare Ganache: Place the chocolate chips in a small bowl. Heat the heavy cream until it just begins to boil, then pour it over the chocolate chips. Let sit for 1 minute before stirring until smooth.
2. Fill Cookies: Spoon or pipe the ganache into the cookie wells. Sprinkle with nonpareils if desired.
3. Set Ganache: Refrigerate the cookies for a few minutes to allow the ganache to set before serving.