Preheat oven to 350°F (180°C). Line baking sheets with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar at medium speed until creamy, about 3 minutes, stopping to scrape sides of bowl. Beat in egg yolks and vanilla.

In a medium bowl, whisk together flour, black cocoa, salt, and baking powder. With mixer on low speed, gradually add flour mixture to butter mixture, beating until combined.

Shape dough into 1-inch balls (about 13 grams), and place 2 inches apart on prepared pans. Using your thumb or a ¼ teaspoon, gently make an indentation in center of each ball.

Bake until set, 9 to 10 minutes. Remove from oven, and press down centers again. Let cool on pans for 10 minutes. Remove from pans, and let cool completely on wire racks. Place Vanilla Buttercream in a piping bag fitted with a large round tip (Wilton #12). Pipe Vanilla Buttercream into center of each cookie. Crush a few cookies, and sprinkle on top of buttercream, if desired.