01 - 
                In a medium bowl, toss together cocoa powder, flour, a pinch of salt, and baking soda. Put it aside.
              
              
              
                02 - 
                Using an electric mixer, whip up the softened butter with the sugar for about 3 minutes, or until it looks nice and airy.
              
              
              
                03 - 
                Crack in one egg at a time, blending well after each. Mix in the vanilla right after.
              
              
              
                04 - 
                Slowly add the dry mix into the wet one, keeping the mixer on its lowest setting. Don’t overmix. You can gently fold in marshmallow after chilling if you'd like a specific swirl.
              
              
              
                05 - 
                Pop the dough in the fridge for at least an hour. This will make shaping easier and keep them from spreading too much when baking.
              
              
              
                06 - 
                Turn your oven on to 175°C (350°F) about 20 minutes before baking, and prepare two trays with parchment or baking mats.
              
              
              
                07 - 
                Scoop out some cold dough with a tablespoon or cookie scoop. Roll them into balls and space them 2 inches apart on the trays. Press a dent in the tops, then spoon a bit of marshmallow fluff inside. You can swirl it around with a toothpick for decoration.
              
              
              
                08 - 
                Bake for 10-12 minutes. The edges should look firm, but the centers will still be nice and soft. Add extra marshmallow halfway through if you want them extra gooey.
              
              
              
                09 - 
                Leave the cookies on the tray for a few minutes to cool down before carefully transferring them to a rack to finish. It’ll help keep the marshmallow fluffy.