In a small bowl, mix together the egg yolk, butter, brown sugar, and vanilla extract.
Then, add in all purpose flour and baking powder and mix well to form a dough.
Mix in 3 tbsp of the chocolate chips and gently fold them in.
Split the cookie dough into two balls. Fold in one marshmallow into the center of each ball of cookie dough. Roll back into two balls.
Add the remaining tablespoon of chocolate chips on top of the cookies.
Place cookie dough balls on a parchment lined baking sheet and bake for about 15-20 minutes at 350 degrees. (Or if you're making one large cookie, it will take 25-30 mins).
Remove the cookies from the oven, and cool at room temperature for at least 10 minutes before eating!
Notes
If you don't want your s'mores to explode (1) Chill the dough for 30-60 minutes . (2) Encase the marshmallow properly by fully enclosing it within the dough (3) Use mini marshmallows or marshmallow bits instead of full-size ones (4) Partially freeze marshmallows for 10-15 minutes before baking to slow down melting. (5) Shape the dough into rounded balls rather than flattening it (6) Use marshmallow fluff instead of whole marshmallows