For the Cookies

1. Preheat oven to 350 degrees Fahrenheit and line baking sheets with parchment paper.
2. In a medium-sized bowl, whisk all purpose flour, baking soda, baking powder, salt, and corn starch.
3. In a large bowl with a hand mixer or in the bowl of a stand mixer fitted with a paddle attachment, cream butter, granulated sugar, and light brown sugar together until smooth, light, and creamy. This should be about 2 minutes on medium/high speed.
4. Beat egg, vanilla extract, and butter extract into the batter, mixing until smooth and combined.
5. Slowly mix dry ingredients into the wet. Scrape down the sides of the bowl as you add the flour mixture. Mix until ingredients are just combined, as overmixing will impact the texture of your cookies.
6. Form dough into slightly larger than 1 tablespoon balls. Place balls at least 2 inches apart on a baking sheet. You will most likely need more than one pan, as you want to make sure the cookies have enough room to spread.
7. Bake cookies for 7-9 minutes or until the tops are JUST set. Cookies should not look wet but should not be golden or light golden brown in color. After removing cookies from the oven, allow them to cool and set on the baking sheet for at least 10 minutes. If you fear you have overbaked the cookies, meaning they look golden around the edges, immediately remove them from the baking sheet to prevent further baking.
8. Let cookies cool before assembling sandwiches.

For the Frosting

1. In a large bowl with a hand mixer or in the bowl of a stand mixer fitted with a paddle attachment, beat butter and powdered sugar until smooth. If using a hand mixer, you will need to use room temperature butter.
2. Add marshmallow fluff and heavy whipping cream, and beat frosting for about 2 minutes until smooth, light in color, and velvety!

For Assembly

1. Place marshmallow filling into a Ziploc bag or piping bag if desired. Doing so will give you greater control and a smoother finish when filling the cookies.
2. Pipe or spread filling onto half of the cookies before placing another cookie on top of the filled cookies.
3. Roll the cookie sandwiches in a bowl of remaining sprinkles for extra decoration.
4. Cover and store at room temperature for up to 4 days.