1. Make the Cookie Dough

1. Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. Cream the butter, brown sugar, and granulated sugar together until light and fluffy.
3. Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
4. In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, and salt.
5. Gradually add the dry ingredients to the wet, mixing until fully combined.
6. Scoop cookie dough onto baking sheets using a medium cookie scoop (about 2 tablespoons). Leave 2 inches between each.
7. Bake for 9–11 minutes. Let cool on the sheet for 5 minutes before transferring to a wire rack.

2. Make the Marshmallow Frosting

1. Beat the butter until creamy.
2. Add powdered sugar, vanilla, and salt. Mix on low until mostly incorporated, then beat on high until fluffy.
3. Add marshmallow fluff and 1 tablespoon cream; beat until smooth. Add extra cream if needed for piping.

3. Make the Chocolate Ganache

1. Place chopped chocolate in a heat-safe bowl.
2. Heat cream and corn syrup in a small saucepan until just boiling. Pour over chocolate.
3. Let sit 2 minutes, then stir until smooth. Let cool until slightly thickened.

4. Make the Pipeable Icing

1. Beat together butter and powdered sugar. Add vanilla and cream.
2. Beat until smooth. Transfer to a piping bag with a small round tip.

5. Assemble the Cookies

1. Once cookies are cool, pipe or spoon a mound of marshmallow frosting on each.
2. Spoon or drizzle ganache over the top. Let set slightly.
3. Pipe a swirl of icing across the top of each cookie to mimic the Hostess design.