Softened butter, to prepare the baking dish and knife
3 envelopes (21 grams) unflavored gelatin
1 cup cold water, divided
1 3/4 cups (350 grams) granulated sugar
3/4 cup (240 grams) light corn syrup
Pinch of kosher salt
2 teaspoons pure vanilla extract
1 teaspoon flavored extract, such as mint (optional)
Food coloring (optional)
1 cup (120 grams) confectioners’ sugar
Lightly grease an 8 x 8-inch baking dish with softened butter.
Dissolve the unflavored gelatin in 1/2 cup of the water in the bowl of your stand mixer fitted with the whip attachment.
In a medium saucepan, cook the granulated sugar, corn syrup, salt, and remaining 1/2 cup water until it reaches 245°F on a candy thermometer. Turn the mixer on low speed and carefully pour in the hot sugar syrup along the side of the bowl, being careful that it doesn't hit the whip attachment.
Once you have all the syrup in the bowl, turn the speed up to high and let it mix for about 10 to 12 minutes, until the marshmallow is light, fluffy, and cooled to nearly room temperature. This is when you add the vanilla, and any additional flavors or color, if using.
Pour the marshmallow into the prepared baking dish and allow to sit for several hours or overnight at room temperature. To cut, dust the marshmallows and work surface generously with confectioners’ sugar. Butter the blade of a knife and cut into your desired shape. Dust with more confectioners’ sugar to prevent from sticking.
The marshmallows will hold their shape and can be stored in an airtight container for a week. You want to make sure they are well coated with confectioners’ sugar or they will stick together.