Preheat oven to 350°F with the racks in the middle. Grease 2 baking sheets with butter or cooking spray.

In a bowl, using a wooden spoon, stir together the flour, cocoa, baking powder and salt. Set aside.

Using an electric mixer, beat the butter and sugar on medium speed until creamy, about 3 minutes. Turn off the mixer and scrape down the bowl with a rubber spatula. Add 1 egg and beat on medium speed until blended. Add the other egg and vanilla and beat until blended.

Turn off the mixer and add the flour mixture. Beat on low speed just until blended.

Using a tablespoon, scoop up a rounded spoonful of dough. Scrape the dough off the spoon into the palm of your other hand. Roll the dough into a ball. Roll some of the balls in crystal sugar until covered. Keep some plain. Place the balls on the prepared baking sheets. Repeat, spacing the balls about 2 inches apart.

Put the cookies in the oven and bake until they are crackled and puffed, 10 to 12 minutes. Using oven mitts, remove the baking sheets from the oven. Immediately place 3 M&M's in the center of each cookie, pressing down firmly so the cookies crack on the edges.

When the cookies have cooled a few minutes, use a firm spatula & transfer the cookies onto the rack and let cool completely. Repeat with the rest of the cookies.