Preheat oven to 350°F. Line two baking sheets with parchment paper, set aside.
In medium bowl, sift or whisk together flour, cocoa powder, baking soda and salt; set aside.
Melt 3/4 cup chocolate chips over a double boiler or in microwave until fully melted through; set aside to cool.
In bowl of stand mixer fitted with paddle attachment, cream together butter and sugar until light and fluffy, about 3 to 5 minutes. Stir in eggs, one at a time, mixing well after each addition. Add in cooled melted chocolate and vanilla extract, stir until well combined. Add flour mixture all at once and slowly stir until just combined. Fold in remaining chocolate chips.
Cover dough with plastic wrap and refrigerate 30 minutes.
Using tablespoon, scoop out dough and place on prepared baking sheets about 2 inches apart. Bake 7 to 8 minutes, leaving cookies slightly under-baked. Remove from oven and immediately place one jumbo marshmallow on top of each cookie. Place pan under broiler on high 1 to 2 minutes to toast marshmallows. Remove and let cool on baking sheets about 5 minutes, then transfer to wire racks to cool completely. Repeat with the remaining cookie dough.
Serve warm or let cool and store at room temperature for up to 2 days.