To make marshmallows at home, combine 3 tablespoons of gelatin (a professional-grade concentrated gelatin would be a good choice, but generally is available only to the food industry) with 1/2 cup of cold water and let it stand for an hour. In the meantime, heat 2 cups of sugar, 3/4 cup light corn syrup, 1/2 cup of water, and 1/4 teaspoon of salt. Bring it to a boil and cook until the firm-ball stage (244°F; 117°C). Remove from the heat and pour it slowly over the gelatin, beating it constantly in a stand mixer. Beat it for a good 15 minutes. When it’s thick but still warm, add 2 tablespoons of vanilla or other flavoring (mint, strawberry, orange, flower water, etc.), if desired. Spread the mixture in a pan that has been lightly dusted with cornstarch. Let it dry for 12 hours, then cut it into squares with scissors dusted with cornstarch, and store them in an airtight container.