Preheat the oven to 375°F and line a baking sheet with parchment paper.

Cream 1/2 cup cool softened butter, 1/2 cup granulated sugar, and 1/2 cup packed light brown sugar only until combined.

Mix in 1 large cold egg and 1 teaspoon vanilla extract.

Add 1 cup plus 3 tablespoons all-purpose flour, 1/2 cup cocoa powder, 1 teaspoon cornstarch, 1/2 teaspoon baking soda, and 1/2 teaspoon salt. Mix only until a soft, thick dough forms.

Scoop 12 portions of 2 tablespoons dough each onto the prepared baking sheet and chill for 15 minutes.

Bake the chilled portions for 11 to 14 minutes, until edges are set and centers look slightly underdone. Cool all 12 cookies completely.

Beat 1/2 cup softened butter, 1 cup powdered sugar, 1 cup marshmallow fluff, 1 teaspoon vanilla extract, and a pinch of salt until fluffy. Pipe or spread filling onto 6 cookies and chill.

Melt 1 cup dark chocolate chips with 1 teaspoon coconut oil in 30-second microwave bursts, stirring until smooth. Spoon chocolate over the remaining 6 cookies and chill for 5 to 10 minutes.

Whisk 1/2 cup powdered sugar, 1 to 2 teaspoons milk, and 1 teaspoon vanilla extract until thick and pipeable. Pipe a white swirl over the 6 chocolate-topped cookies and chill for 5 to 10 minutes.

Press each chocolate-topped cookie onto a filled cookie. Chill the 6 cookie sandwiches for 20 minutes before serving.