Preheat oven to 400 degrees F.
Cream butter, brown sugar, and white sugar together. Add egg and vanilla. Mix until incorporated.
Add dry ingredients (cocoa powder, salt, baking soda, flour). Mix until just blended.
Scoop dough into 1 tablespoon portions. I love using my size 70 scoop.
Flatten dough into a circle. Place marshmallow in center and form dough around marshmallow. Make sure marshmallow is fully sealed or it will ooze out when baked.
Roll in sugar and place on cookie sheet lined with parchment paper or a silicone mat (up to 15 per sheet).
Bake for 4 1/2 to 5 minutes. Do not overbake or marshmallow will melt or ooze out of cookie. Let cool and enjoy.