In a 4-quart saucepan, melt butter and 1 ⅓ cup semi-sweet chocolate chips on medium-low heat, constantly stirring so it doesn’t burn.
Remove from the heat and let cool for 5-10 minutes, until lukewarm.
Mix the flour, cocoa powder, baking soda, and salt in a medium bowl.
Whisk sugar, eggs, and vanilla extract into the saucepan with the melted chocolate and butter.
Stir the flour mixture into the chocolate mixture until a dough forms.
Fold in the remaining 1 cup of semi-sweet chocolate chips.
REST DOUGH:
Let dough rest for 25-30 minutes (see notes). The dough will stiffen into a formable dough.
PREP OVEN AND COOKIE SHEETS:
Move oven rack to the middle positions and preheat the oven to 350 degrees F.
Line a cookie sheets with parchment paper or silicone baking mats.
SCOOP AND FILL DOUGH:
Scoop with a 2-inch cookie scoop 2 tablespoons of dough and roll the dough into a ball.
Press your thumb into the center of the ball, create a bowl in the dough and place 3 marshmallows in the center.
Fold over the dough to cover the marshmallows.
Pat the dough into a ball, be careful to keep the dough ball marshmallow side up.
BAKE THE COOKIES:
Place the dough balls marshmallow side up on a baking sheet, and bake one cookie sheet at a time for 9-10 minutes. They will appear dry around the edges but slightly moist in the center (not gooey or wet).
Repeat baking with the remaining cookie sheets.
COOL AND FINISH:
Cool the cookies on the cookie sheet for 5-6 minutes before removing them gently to cool on a cooling rack. Cool on the wire rack completely, another 55 minutes.
Notes
Be sure the melted butter and chocolate is cooled, or the chips added later can melt and make the cookie spread while baking.
Place dough in the fridge for 25 minutes to stiffen if you live in a warmer climate.
If you place the dough balls marshmallow down, the marshmallow can melt and pool onto your cookie sheet.
Do not overbake. The marshmallow will melt and become hard, making a hard crispy cookie.
Underbaking will result in a fragile cookie, so will eating them before they cool completely.