How to Make It
Make the cookies In the bowl of a stand mixer fitted with the paddle, beat the sugar with the butter at medium speed until light and fluffy, about 2 minutes. Beat in the egg yolk and vanilla until incorporated, about 2 minutes. At low speed, beat in the flour and salt until the dough just comes together. Turn the dough out onto a work surface and pat into 3 disks.
Lightly dust a sheet of parchment paper with flour. Working with 1 disk at a time, roll out the dough 1/4 inch thick. Dust lightly with flour and top with another sheet of parchment. Transfer to a baking sheet and repeat with the remaining disk of dough, stacking the dough between sheets of parchment. Refrigerate for 1 hour.
Preheat the oven to 350°. Line 3 large rimmed baking sheets with parchment paper. Using a 1 1/2-inch round cookie cutter, stamp out the cookies as close together as possible; transfer to the prepared baking sheets 1 inch apart. Gather the scraps and stamp out more. Freeze for 30 minutes.
Bake the cookies for 10 to 12 minutes, until crisp and lightly golden. Transfer to a wire rack to cool completely, about 45 minutes.
Make the marshmallows and glaze In the bowl of a stand mixer fitted with the whisk, beat the egg whites at medium-high speed until soft peaks form, about 5 minutes. In a small bowl, sprinkle the gelatin over 2 tablespoons of cold water and let stand for 5 minutes, until dissolved. Meanwhile, in a medium saucepan, combine the granulated sugar with the corn syrup and ¼ cup of water. Cook over moderately high heat, without stirring, until the syrup reaches 235° on a candy thermometer, about 6 minutes. Remove the syrup from the heat and stir in the gelatin. Gradually pour the hot syrup into the egg whites in a steady stream and beat at medium-high speed until stiff and glossy, about 15 minutes. Scrape the marshmallow into a resealable plastic bag and using scissors, snip off a small corner.
Line 2 large rimmed baking sheets with parchment paper and top with wire racks. Pipe a dollop of marshmallow on each cookie, lifting as you pipe to create the shape of a ghost. Let the marshmallows stand at room temperature for 2 hours, then lightly dust with confectioners’ sugar.
Meanwhile, combine the white chocolate and the oil in a medium heatproof bowl set over a saucepan of simmering water; stir until the chocolate is melted and smooth. Remove from the heat and keep warm. Carefully drop each cookie into the melted chocolate, then lift it out with a fork, letting the excess drip off. Transfer to the prepared rack. Using your hands or tweezers, place two mini chocolate chips on the ghost to mimic eyes. Repeat with the remaining ghosts. Refrigerate for 30 minutes, until the chocolate is firm.