Preheat the oven to 375° F. Have ready a 10-inch tart pan.
In the bowl of an electric mixer, cream the butter and confectioners' sugar until light and fluffy, 4 to 5 minutes. Add the vanilla and mix to combine.
Add the flour and salt and mix just until incorporated. Press the dough into the tart pan, and trim away the excess. Freeze until thoroughly chilled, 15 to 20 minutes.
Dock all over with a fork. Place a piece of parchment over the crust and fill with pie weights. Bake until the crust is lightly golden brown, 15 to 17 minutes. Remove the parchment and pie weights, and brush the partially-baked dough with egg wash. Return to the oven and bake until fully baked, 5 to 10 minutes more. Cool completely.
Place the chopped chocolate in a small, heat-safe bowl. Bring the cream to a boil over medium heat, and pour over the chocolate. Let sit for 15 seconds, then mix until smooth. Pour the chocolate into the cooled crust and spread into an even layer over the base. Let set completely.
In a small bowl, bloom the gelatin in 2 tablespoons cool water for 5 minutes. Place the egg whites in the bowl of an electric mixer fitted with the whip attachment.
In a medium pot, bring 1/4 cup water, the sugar, and the corn syrup to a simmer over medium heat. Stir until the mixture comes to a boil, then leave undisturbed. Boil until the mixture reads 235° F on a thermometer.
When the sugar reaches 235° F, begin to whip the egg whites. The idea is to have them at soft peaks when your sugar reaches 245° F. Melt the gelatin in the microwave, 15 to 20 seconds.
With the mixer running, pour the sugar syrup in a slow, steady stream into the egg whites. Add the vanilla and the melted gelatin. Continue to whisk on medium-high speed until the mixture is cool to the touch, 7 to 8 minutes.
Pour the marshmallow mixture into the crust and used a greased offset spatula to form a sort of dome shape (like a Mallomar). Work quickly, as the gelatin will start to set. Once you’re happy with the shape, let the mixture set completely.
To make the topping, melt the chocolate and coconut oil together over a pot of simmering water.
Once the marshmallow is set, use a ladle to gently pour the chocolate over the marshmallow in an even layer. Some will pool around the edges—that’s okay, but try to minimize it as much as possible. Let the whole tart set at room temperature for 15 to 30 minutes before slicing and serving.
I always carry three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's pie. My first cookbook, The Fearless Baker, is out on October 24, 2017.