Preheat oven to 375 F. Line two baking sheets with parchment paper.

Sift flour, cocoa powder, baking soda, and salt into a medium bowl.

In a large bowl, beat the butter, brown sugar, and vanilla together until fluffy. Add the eggs, one at a time, beating after each addition until combined.

On low speed, add the flour mixture to the butter mixture and mix until just combined.

Stir in the m&ms and white chocolate chips.

Drop dough by the tablespoon onto the prepared baking sheets. Leave about 2 inches between each cookie. Press a couple m&ms and white chocolate chips onto the top of each cookie. Sprinkle with flaky salt if desired.

Bake for about 12 minutes, until puffy but still soft and slightly shiny in the middle.

Let cookies rest on the baking sheet for 2 minutes before transferring to a wire rack to cool completely.