Preheat oven to 350 degree. Line 13x9 in baking dish with parchment paper.
In a large bowl mix butter and sugars until completely combined, then mix in the egg and vanilla.
In a separate bowl combine flour, cocoa powder, baking soda, salt and cornstarch.
Combine dry ingredients with the wet ingredients until fully formed into a dough. Mix in the chocolate chips and press into the baking dish evenly. Bake for 15 to 18 minutes.
As soon as you pull the cookie bars out of the oven gently press marshmallows into the top of the bars. Cool in the baking dish for 10 to 15 minutes, then transfer to a cutting board to finish cooling.
Once cool drizzle with melted chocolate and cut into bars.
Store in an airtight container.