Preheat oven to 350°F. Grease half of a 12-cup muffin tin with non-stick cooking spray (this recipes makes 6 cups). 

In a large mixing bowl, whisk together melted butter, sugars, milk, and vanilla until creamy. Add flour, cornstarch, salt, baking soda, and powder. Use a wooden spoon or rubber spatula to fold dry ingredients into wet until a soft dough forms. Fold in graham cracker and chocolate chips.

Scoop 2 ½-3 tbsp dough into each muffin cup and lightly press dough down so it sits evenly in the bottom (do not press up the sides yet). 

Bake for 14-15 min until edges are set. The center should still look soft and slightly underbaked. Remove from oven and immediately use the bottom of a ¼ cup measuring cup to gently press into the center of each cookie to form a cup shape. 

Cool cookie cups for 15 minutes before carefully removing. Cool completely before adding filling.

Make the ganache: Add chocolate chips and butter to a heat proof bowl. Pour milk into a saucepan and bring to a low boil over medium-high. Pour hot milk over chocolate chips and let sit for 3 minutes (do not stir yet).

After 3 minutes, whisk together chocolate chips, butter, and milk until a creamy ganache forms. Divide ganache amongst the 6 cooled cookie cups. Transfer cookie cups to the fridge to set for 15 minutes.

Top cookie cups with mini marshmallows, crushed graham cracker, and flaky sea salt before serving. For that extra campfire touch, toast mini marshmallows with a kitchen torch.