In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar on medium-high speed until creamy, about 2 minutes. Scrape down bottom and sides of bowl. Add the eggs and vanilla and beat until combined. On low speed, gradually beat in the flour mixture. Dough will be thick and slightly sticky. Cover bowl with plastic wrap or foil and place dough in fridge for 30 minutes to firm up before proceeding.

Bake for 11 minutes, or until just set. While the cookies are baking, clean your cookie scoop. Immediately after removing from the oven, use the bottom of the scoop to push down the center of each cookie, holding for about two seconds before carefully lifting up. The middle of each cookie will be very slightly underdone. Wipe scoop clean as needed. Allow to cool completely before adding filling.

In the bowl of an electric mixer fitted with the paddle attachment, beat the marshmallow creme and butter on low speed until combined. Scrape down the sides and bottom of the bowl. Mix in the powdered sugar, vanilla, and salt on low speed until combined, then increase to medium-high speed and beat until very smooth, about 2 minutes. Transfer the filling to a piping bag or zip-top bag with the one corner snipped off. Set aside while you make the ganache.

Place chopped bittersweet chocolate and powdered sugar in a heatproof bowl.

Pour the heavy cream and corn syrup into a small saucepan set over medium-high heat and bring just to a boil. Pour over the chocolate. Cover and let stand for 5 minutes before vigorously whisking until completely smooth.

Allow to cool and thicken slightly for 5 minutes before proceeding.

Pipe the marshmallow filling into the center of each cooled cookie, just to the brim and level with the cookie. If your filling is sticking up, lightly wet your index finger and flatten evenly.

While the ganache is still warm, pour about 1 teaspoon onto the top of a cookie, then use an icing spatula or the back of a metal spoon to gently smooth over the marshmallow filling and close to the edge of the cookie. Repeat with the remaining cookies. If your ganache thickens too much at any point, place in microwave for about 5-10 seconds to thin slightly, and rewhisk. Let cookies set for about 15 minutes before topping with icing.

In a small heatproof bowl, melt the white chocolate chips in 15-second increments, stirring after each, until just melted. Transfer to a piping bag fitted with a small round tip.

Pipe loopy circles of white icing across the center of each cookie. Let the cookies dry for at least one hour.

Store cookies in an airtight container at room temperature for up to 3 days or in the fridge for up to 5 days. If chilled, let set at room temperature for about 30 minutes before enjoying.