In a medium bowl, sift together flour, cornstarch, baking powder, baking soda, and salt, whisking to combine. Set aside.
Beat slightly softened butter with both sugars in a stand mixer on medium-low for 3–4 minutes until creamy and pale.
Add first cold egg, beat until incorporated. Add second egg and vanilla, mix until combined. Turn mixer off.
Add dry ingredients all at once to the mixer. Mix on low until just combined. Fold in chocolate chips.
Refrigerate dough briefly while preheating oven to 190°C (375°F). Line two baking sheets with parchment paper.
On each baking sheet, place 4 biscuits, evenly apart. Top each biscuit with 1-2 chocolate squares, then top with a marshmallow. (you may end up with only 5 cookies, depends how much dough you use for each)
Weigh out 140–150g dough per cookie. Flatten into a disc, then wrap over each marshmallow topped biscuit, over the top and sides (not the bottom). Pinch to seal any cracks that show up so that the cookie dough is totally covering the marshmallow. Watch the video in the recipe card for a visual demonstration.
Garnish with small chocolate covered biscuits or extra chocolate chips.
Bake 13–14 minutes until edges are golden and center is still soft. Check at 11 minutes.
Cool on baking sheet for 10 minutes before eating, then enjoy while still warm.