Preheat oven to 350 F. Line baking sheets with parchment paper or silicone baking mat.
Melt butter in a heavy saucepan or in a microwave-safe bowl.
1 cup butter
Add sugars to a large bowl.
1 ¼ cups brown sugar, ¼ cup granulated sugar
Pour melted butter over sugars.
Stir to combine butter and sugars. Let stand 5 minutes.
Add egg and egg yolk; stir to combine.
1 large egg, 1 egg yolk
Stir in vanilla extract.
1 teaspoon vanilla extract
Add flour, cocoa powder, baking soda, and salt. Stir until just combined.
2 ½ cups sifted all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon baking soda, ½ teaspoon salt
Stir in chocolate chunks and marshmallow bits.
12 ounce package semisweet chocolate chunks, 1 cup marshmallow bits3
Chill the dough for at least 30 minutes to give the ingredients a chance to meld.
Using a medium cookie scoop (1.5 tablespoon), drop scoops of dough 2 inches apart on prepared cookie sheet.
Bake for about 8-12 minutes. The cookies should look set at the edges and slightly underdone in the center. Slightly underbaking the cookies will help them to stay soft.
Let cool on cookie sheet for 5 minutes. Remove to wire rack to finish cooling.
Notes
Properly measuring flour makes a huge difference! To properly measure the flour, sift the flour to break it up. Lightly spoon it into the measuring cup and level.
Marshmallow bits are freeze-dried marshmallow pieces. They are usually sold next to the regular marshmallows.