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In a large bowl, cream butter until it is light and fluffy, about 1 minute. Add in brown sugar and white sugar and beat until well combined, scraping down the sides of the bowl as necessary.
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Next, add in the egg, beating well. Add in vanilla extract and salt; mix.
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Sift together flour, baking soda, corn starch, and espresso powder. Gradually add the dry ingredients into the wet ingredients, stirring until just combined.
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Add in chocolate chips. Stir.
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Chill dough for about 30 minutes.
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Preheat oven to 350 degrees and line two baking sheets with parchment paper or grease with cooking spray.
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Take about 2 Tbsp of dough and flatten it slightly in your hand. Stick 2-3 marshmallows in the center, and fold the cookie dough around the marshmallows and form it into a ball. Make sure none of the marshmallows can be seen in the dough. Repeat until all dough is gone (dough should make 12 large cookies).
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Place dough onto cookie sheets, about 2-3 inches apart.
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Bake for 15-16 minutes, or until cookies are barely turning golden brown (in order for the cookies to remain soft after being in the freezer, they need to be slightly underdone).
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If desired, add a couple more marshmallows on top of each cookie as they cool.
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Allow to cool completely.