Preheat the oven to 350. Add the butter and brown sugar to the bowl of a stand mixer. Mix for 2-3 minutes until creamier and smooth. Add eggs and vanilla and mix until just combined.
Add flour, cornstarch, baking soda, and salt. Mix until just combined.
Add both kinds of chocolate chunks (2 cups total) and mix until the chocolate is evenly distributed.
For larger cookies, scoop out 1/2 cup of dough per cookie. Shape them into a ball and then flatten slightly with your hand. For smaller cookies, scoop out 1/4 cup of dough.
For 1/2 cup cookies, bake for 8 minutes. Flatten the center of each cookie slightly with a spoon and add 1-2 tbsp of marshmallow fluff on top. Transfer back to the oven and bake another 3-4 minutes until cookies are just set.
For 1/4 cup cookies, bake for 5 minutes, then add the marshmallow fluff, then bake another 3-4 minutes until cookies are just set.
After the cookies are finished cooking, take a creme brulee torch and toast the marshmallow fluff until golden brown. If you don't have a creme brulee torch, you can set the oven to broil, and broil the cookies for 30-60 seconds until the marshmallow fluff is toasted. If you are doing this, keep a close eye on the cookies so they don't burn!
Once the cookies cool completely, melt the 1/4 cup of chocolate chunks. Melt in the microwave, 30 seconds at a time, stirring between intervals. If the chocolate isn't melting evenly, add 1 tsp of coconut oil or vegetable oil. This will help the chocolate melt more evenly.
Drizzle the melted chocolate onto each cookie, then add crushed graham cracker on top. Enjoy!!
Keep any leftover cookies in a sealable container.
