Preheat the oven to 375 F. Line two baking sheets with parchment paper. Set aside.

In a medium bowl, sift together flour, cocoa powder, baking soda, and salt. Set aside.

In a large bowl, beat the butter until creamy. Add the sugar and beat until light and fluffy. Add the egg and beat until well combined.

Gradually add the flour mixture to the butter mixture, beating until well combined. The dough will be a bit crumbly.

To make the shaped cookies, press half of the dough into a large disk on a lightly floured surface. Using a rolling pin and firm pressure, press/roll the dough into a layer about ¼ inch thick. Cut out desired number of shapes using a number 2 shaped cookie cutter and a small Mickey shaped cookie cutter.

Place cut cookies onto prepared baking sheets and coat. Flip some of the number 2 shaped cookies so that the glitter side will be on the outside when sandwiched. Coat each cookie with edible glitter using a small pastry brush.

To make the round cookies, roll a spoonful of dough into a small ball and place onto prepared baking sheets, leaving space for the dough to expand. Repeat until dough is gone.

Place the edible glitter onto a small plate. Dip the base of a flat bottom glass into the glitter and then press it down onto a dough ball to flatten. Repeat until all balls are flattened.

Bake for 8-10 minutes, just until the top of the cookie is firm. Cool cookies on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.

Place the egg whites and cream of tartar into a heat-safe bowl. Beat on low speed until the eggs are foamy. Set aside.

Place the sugar, corn syrup, and water into a small saucepan over low heat. Stir frequently until the sugar has dissolved. Turn the heat to medium high and cook until the syrup reaches 240 F.

Steadily pour the sugar syrup into the egg whites, beating constantly. Beat on medium high speed until the mixture is very thick, fluffy, and glossy, about 8-10 minutes. Beat in the salt and vanilla.

Transfer the marshmallow fluff to a large piping bag.

Once the cookies have cooled, pipe a generous amount of marshmallow fluff to the underside of 12 cookies. Place the remaining cookies, sparkle side up on top to create 24 sandwich cookies.