Combine flaxseed and water. Let it sit to become your flawless egg substitute.
In one bowl, whisk flour, baking soda, and salt.
In another, cream butter and both sugars until light and extra sassy.
Stir in the flax egg and vanilla. Mix well.
Add dry ingredients slowly, forming thick, irresistible cookie dough.
Fold in chocolate chips like you're hiding treasure.
Chill dough in the fridge for 10 minutes—trust me.
Scoop dough into balls and flatten slightly on parchment-lined tray.
Bake at 350°F (175°C) for 12 minutes until golden around the edges.
Cool on rack while you resist temptation.
For the filling: whisk marshmallow fluff, espresso, powdered sugar, and salt until smooth.
Spread 1 tbsp filling on one cookie, top with another. Repeat. Try not to eat them all instantly.