Prep Dry Ingredients: Whisk together flour, cocoa powder, baking powder, and sea salt in a mixing bowl. Set aside.

Cream Wet Ingredients: In a large bowl, cream together butter, brown sugar, and white sugar until smooth. Add egg yolks and vanilla, mix until fluffy.

Combine Mixtures: Gradually add the flour mixture to the wet ingredients, mixing on low speed until just combined.

Shape Dough: Scoop dough into 32 rounded tablespoon portions, roll into balls, and make an indent in the center of each ball. Chill for 1 hour.

Bake Cookies: Preheat oven to 350°F. Bake cookies for about 9-11 minutes until edges are firm; cool on sheets.

Make Ganache: Combine chopped chocolate and heated heavy cream in a bowl, stir until smooth.

Fill Cookies: Once cooled, fill the indents with ganache, then chill for 15-20 minutes to set before serving.