Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper, letting the oven heat while you work.

In a large bowl, beat 1 cup unsalted butter, 1 cup granulated sugar, and 1 cup packed light brown sugar on medium speed for 2 minutes until pale and fluffy.

Add 2 large eggs and 1 teaspoon vanilla extract and beat until the mixture is glossy and smooth.

Whisk together 2 cups all-purpose flour, ½ cup unsweetened cocoa powder, 2 teaspoons cornstarch, 1 teaspoon baking soda, and ½ teaspoon salt in a medium bowl.

With the mixer on low, add the dry ingredients to the wet in two additions, mixing just until combined.

Fold in 1 cup semisweet chocolate chips and 1 cup mini marshmallows using a rubber spatula.

Scoop 2-tablespoon balls of dough onto prepared sheets, spacing them 2 inches apart, and optionally sprinkle with pink finishing sugar.

Bake for 10–11 minutes, until the edges are set and the centers are still soft and glossy.

Let cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.