In a large mixing bowl, whisk together 1 ¾ cups all-purpose flour, ⅔ cup Dutch process cocoa powder, ½ teaspoon salt, and 1 teaspoon baking powder until well combined.

In a separate bowl, cream ¾ cup softened unsalted butter, ½ cup light brown sugar, and ½ cup granulated white sugar using an electric mixer on medium speed until fluffy and pale, about 3 minutes.

Blend in 2 egg yolks and 1 teaspoon vanilla until light and smooth, then gradually mix in the dry ingredients until just combined.

Scoop the chocolate dough into 34 equally sized portions, rolling each into smooth balls about 1 inch in diameter.

Place the dough balls on a parchment-lined baking sheet, spaced 2 inches apart. Using a ¼ teaspoon measuring spoon, gently press into the center of each ball to create an indent.

Chill the cookie shapes in the refrigerator for at least 1 hour.

Preheat your oven to 350°F (175°C). Once the dough balls have chilled, bake them for 9 to 11 minutes, or until the edges are just firm but the centers remain soft.

Remove the baking sheet from the oven and, while the cookies are still warm, re-press the indents slightly with your teaspoon for a deeper pocket.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

To prepare the ganache filling, heat ⅔ cup heavy whipping cream in a small saucepan until it just begins to boil.

Remove the pan from the heat and pour the hot cream over 1 cup of semi-sweet chocolate chips in a heatproof bowl. Let it sit for 2-3 minutes to soften, then stir gently until smooth and glossy.

Once the cookies are cooled, use a spoon to fill each indent with the prepared chocolate ganache. Optionally sprinkle with colorful nonpareils.

Allow the ganache to set by placing the filled cookies in the refrigerator for about 30 minutes.