Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
In a medium bowl, whisk together 2 1/2 cups all-purpose flour, 1/4 cup Hershey’s Special Dark Cocoa Powder, 1/4 cup Dutch-processed cocoa powder, 1 teaspoon baking soda, and 1 teaspoon kosher salt. Set aside.
In a microwave-safe bowl, melt 6 oz milk or semisweet chocolate chips with 4 oz (half) of the unsalted butter. Heat in 20-second intervals, stirring between each, until smooth. Let cool slightly.
In a large mixing bowl, use an electric mixer to cream the remaining 4 oz softened butter with 2 cups granulated sugar until light and fluffy, about 3-4 minutes. Add 2 eggs one at a time, beating well after each addition. Stir in 1 teaspoon vanilla extract.
Slowly pour the cooled melted chocolate and butter mixture into the creamed sugar and eggs, mixing until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until combined. Be careful not to overmix to keep cookies tender.
Gently fold in 1/2 cup milk chocolate M&M candies, 1/2 cup white chocolate peppermint M&M candies, and 1/2 cup white chocolate chips.
Using a cookie scoop or tablespoon, drop rounded balls of cookie dough onto prepared baking sheets, spacing about 2 inches apart to allow spreading.Bake for 10-12 minutes, or until the edges are set but centers still look slightly soft. The cookies will firm up as they cool. Allow cookies to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely. Enjoy warm for gooey chocolate or room temperature for more structure.