Preheat oven to 375 degrees Fahrenheit. Lightly spray a cooking sheet with non-stick cooking spray.
Mix the flour and baking soda in a small bowl and set aside.
Mix peanut butter, butter, and brown sugar in a medium bowl with a spoon until smooth.
Add the egg and the vanilla; beat until smooth.
Add the flour mixture and stir until combined.
Roll the dough into 1-inch balls and place on cookie sheet.
Bake 8-10 minutes.
Remove from oven and allow to cool for 10 minutes - then place in refrigerator and allow to cool fully for another 20 minutes.
FOR THE FILLING
In a microwave-safe bowl heat chocolate chips and butter for about 1 minutes; just until melted. Allow the mixture to sit for about 30 minutes or until slightly stiff.
Heat the marshmallow cream in a microwave for 10 seconds to soften.
Spread marshmallow on a cookie, chocolate on another and sandwich together.
Store cookies in a covered container in the fridge.
Notes
Chilling the Dough: After baking the cookies, it's essential to let them cool for 10 minutes at room temperature before transferring them to the refrigerator. Chilling the cookies helps them firm up and makes assembly easier when adding the filling.
Consistency of Filling: Ensure that the chocolate mixture is only slightly stiff before spreading it on the cookies. If it becomes too firm, it may be challenging to spread evenly. Similarly, microwave the marshmallow cream for just 10 seconds to soften it, making it easier to spread.
Storage: Since the filling contains marshmallow cream, it's best to store the smookies in a covered container in the refrigerator. This helps maintain their texture and prevents the filling from becoming too soft or sticky at room temperature.