Get Ready: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper and lightly spray them with nonstick cooking spray – this helps prevent sticking, even with parchment!
Mix Dry Ingredients: In a large mixing bowl, gently whisk together the powdered sugar, salt, unsweetened cocoa powder, and cornstarch until well combined.
Add Wet Ingredients: Pour in the egg whites and vanilla extract. Stir steadily until a thick, rich dough forms. Don't worry if it looks like there isn't enough liquid at first; just keep stirring carefully, and it will come together beautifully. This thick batter is key to super fudgy cookies!
Fold in Chocolate: Carefully fold in the chocolate chunks or chips until they are evenly distributed throughout the dough.
Form & Decorate: Scoop about two tablespoons of dough for each cookie, shaping them into balls. Place them a few inches apart on your prepared baking sheets. Gently press 5 to 7 Valentine-themed M&Ms (or other candies) onto the top of each cookie.
Bake to Perfection: Bake for 11 to 13 minutes, or until the cookies have mostly lost their glossy sheen and the edges are set. A slight gloss in the very center is perfectly fine and means extra fudginess! Avoid overbaking to keep them soft and chewy.
Cool & Enjoy: Let the cookies cool completely on the baking sheets before carefully removing them with a spatula. This step is important as they firm up as they cool. Store any leftover cookies in an airtight container at room temperature for up to three days.