In a large mixing bowl, beat the butter, sugars, and salt together
with an electric mixer on medium speed until well combined. Add the
egg, egg yolk, and vanilla; beat until incorporated.
In a medium bowl, whisk the flour and corn starch together. Add it to
the butter mixture and beat until fully incorporated. Gather the
dough into a ball, cover with plastic wrap, and refrigerate for at
least 2 hours.
Preheat the oven to 350 F. Scoop 1 tb sized pieces of dough and roll
them into balls. Place the balls of dough 2" apart on a baking sheet
lined with parchment paper or silicon mat. Bake 10 to 12 minutes or
until the edges of the cookies are lightly browned. Cool on the pan
for 5 minutes, then transfer to a wire rack to cool completely.
Filling:
Combine the water, corn syrup, and sugar in a small saucepan fitted
with a candy thermometer. Bring to a boil and cook to "soft-ball"
stage, about 235 F.
Meanwhile, in a large bowl, beat the egg white on medium speed until
soft peaks form.
Sprinkle the gelatin over the water and set aside to soften. Once the
syrup reaches 235 F, add in the softened gelatin and mix until fully
dissolved. With the mixer running on low, slowly pour the syrup into
the beaten egg white. Add the vanilla. Turn the mixer to medium-high
speed and continue to beat until stiff. (This may take 3 to 5
minutes.)
Transfer the marshmallow to a pastry bag fitted with a round tip.
Pipe a large kiss of marshmallow on the bottom side of half the
cookies. Top with a second cookie to form a sandwich. Refrigerate for
30 minutes.
Glaze:
Add chocolate and oil to a heatproof bowl and set it over a pot of
barely simmering water. Stir constantly until chocolate is melted and
smooth. Working with one cookie at a time, use a fork to dip it into
the bowl of melted chocolate. Flip it over to fully coat the cookie
in chocolate. Tap off any excess and let it drip back into the bowl.
Place the cookies on a wire rack set over a large baking sheet.
Refrigerate until chocolate is set.
Keep cookies stored in the refrigerator until ready to serve. Cookies
with keep for up to 3 days stored in an airtight container.
