Step 1

In the bowl of a stand mixer fitted with the whisk attachment, stir together ½ cup water and the gelatin powder. Let sit for 10 minutes to bloom.

Step 2

In a small pot fitted with a candy thermometer, combine the granulated sugar, corn syrup, and remaining ½ cup water. Cook over medium-high heat, without stirring, until the temperature reaches 240°F (115°C), 15–20 minutes.

Step 3

Turn the mixer speed to medium-low and slowly drizzle the sugar syrup into the bloomed gelatin. Increase the mixer speed to high and whip until the mixture is opaque white and doubled in volume, 8–10 minutes. Add the salt and vanilla and beat for 2 minutes more.

Step 4

Divide the marshmallow mixture evenly between 3 medium bowls. Add the red food coloring to one bowl, yellow food coloring to another, and blue to the last and stir until each bowl is evenly colored.

Step 5

In a small bowl using a fork or whisk, stir together the cornstarch and powdered sugar.

Step 6

Use a pastry brush to evenly grease a nonstick 8-inch square pan with the vegetable oil. Use a small sieve to dust the pan with some of the cornstarch and powdered sugar mixture. Tilt the pan to coat evenly, then tap out any excess.

Step 7

Using ice cream scoops, alternate scooping each color marshmallow into the prepared pan, working quickly so the marshmallow does not begin to set. Swirl the colors together using a skewer or knife. Dust the top with more of the powdered sugar mixture.

Step 8

Let the marshmallows at room temperature, uncovered, for at least 4 hours, up to overnight.

Step 9

Run a butter knife or fish spatula around the edges of the pan to loosen the marshmallow, then invert onto a cutting board. Cut into 9 equal squares. Dust the cut marshmallows all over with more of the powdered sugar mixture to prevent sticking. Store in an airtight container in a cool, dry place for up to 1 month, or freeze for up to 2 months.

Step 10

Make the s’mores: Preheat the oven to 300°F (150°C).

Step 11

Place the marshmallows between chocolate or honey graham crackers with a piece of milk or white chocolate. Toast in the oven for 1–2 minutes, until the marshmallows are just softened and the chocolate is beginning to melt.

Step 12

Enjoy!