Set your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat. This helps prevent sticking and promotes even baking.

In a medium bowl, whisk together:

1 cup all-purpose flour

½ teaspoon baking soda

¼ teaspoon salt

⅓ cup unsweetened cocoa powder

This combo is the base of that rich, fudgy cookie texture we’re after.

In a large mixing bowl, use a hand mixer or stand mixer to beat together:

½ cup softened unsalted butter

½ cup granulated sugar

½ cup packed brown sugar

Beat for about 2 minutes, until the mixture is light and fluffy. This step adds air, helping the cookies bake up soft and tender.

Mix in:

1 large egg

1 teaspoon vanilla extract

Blend until smooth and well combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed. The dough will be thick—that’s exactly what you want for chewy cookies!

Using a spatula or spoon, gently stir in:

1 cup semi-sweet chocolate chips

1 cup mini marshmallows

½ cup chopped nuts (optional)

Make sure the goodies are evenly distributed throughout the dough.

Drop heaping tablespoons of dough onto your prepared baking sheet, spacing them about 2 inches apart. These cookies spread slightly, so give them room to shine.

Pop them in the oven for 9–11 minutes, or until the edges are set but the centers still look slightly soft. The marshmallows should be puffed and lightly golden.

Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack. They’ll firm up as they cool but stay gooey in the center.