Chocolate-Dipped Salted Caramel Marshmallows

Brush the bottom and sides of a 9-inch square baking pan with vegetable oil. Using a small, fine-mesh sieve, dust the pan generously with confectioners' sugar, knocking out any excess.

Put 2/3 cup water in the bowl of a stand mixer fitted with the whisk attachment.

Sprinkle the gelatin into the bowl and stir briefly to make sure all the gelatin is in contact with water.

Let soften while you make the sugar syrup.

In a small saucepan, bring 1/2 cup water to a simmer and keep hot, covered. Meanwhile, in a heavy 3- to 4-quart saucepan, combine the granulated sugar and 1/2 cup water and place over moderate heat, stirring until the sugar is dissolved. Once the sugar is dissolved, stop stirring and bring the syrup to a boil, washing down the sides of the pan occasionally with a pastry brush dipped in cold water. Boil until the syrup begins to caramelize. Continue cooking, swirling the pan gently once or twice, until the syrup is a deep golden caramel color.

Remove the pan from the heat, and standing back, carefully add the 1/2 cup hot water—it will bubble up and steam.

Whisk the caramel until smooth, then whisk in the corn syrup and salt. Return the pan to moderate heat and bring the sugar syrup to a boil. Put a candy thermometer into the boiling sugar syrup and continue boiling (the mixture may foam up, so turn the heat down slightly if necessary), without stirring, until the thermometer registers 240°F (soft-ball stage).

Remove the pan from the heat and let stand briefly until the bubbles dissipate slightly.

With the mixer on low speed, pour the hot syrup into the softened gelatin in a thin stream down the side of the bowl. Gradually increase the mixer speed to high and beat until the marshmallow is very thick and forms a thick ribbon when the whisk is lifted, about 5 minutes. Beat in the vanilla.

Scrape the marshmallow into the prepared pan (it will be very sticky) and use wet fingertips to spread it evenly and smooth the top.

Let stand, uncovered, at room temperature until the surface is no longer sticky and you can gently pull the marshmallow away from the sides of the pan with your fingertips, at least 4 hours or overnight.

Dust a cutting board with confectioners' sugar. Use a rubber spatula to pull the sides of the marshmallow from the edge of the pan (use the spatula to loosen the marshmallow from the bottom of the pan if necessary) and invert onto the cutting board. Dust the top with confectioners' sugar.

Brush a long thin knife or a chef's knife with vegetable oil and dust with confectioners' sugar to prevent sticking; continue dusting the knife as necessary.

Cut lengthwise into 8 strips, then crosswise into eighths, to form a total of 64 squares. (For larger marshmallows, cut lengthwise into 6 strips, then crosswise into sixths, to form a total of 36 squares.) Coat marshmallows, one at a time, in confectioners' sugar, using a pastry brush to brush off any excess.

In a dry metal bowl set over a pan of barely simmering water, melt the chocolate and shortening, stirring until smooth. Working with one marshmallow at a time, brush the marshmallow again to remove any excess confectioners' sugar then dip one smooth side about 1/4 inch into the chocolate to coat it, scraping any excess chocolate on the edge of the bowl.

Sprinkle the chocolate-covered side of the marshmallow with a few flakes of sea salt, then transfer, chocolate side up, to a wire rack set over a baking sheet and let stand until the chocolate is set. DO AHEAD: Marshmallows—without chocolate—can be stored, layered between sheets of wax paper or parchment in an airtight container in a dry place at cool room temperature, for 1 month. Chocolate-dipped marshmallows can be stored in the same manner, but will only keep for about 2 days.