With an almost unlimited supply of packaged marshmallows on the market, this is a question we never expected to receive. But before the explosion of food processors (the conglomerates, not the machines), people did make marshmallows at home. In fact, the 1975 edition of Joy of Cooking (Canada, UK) includes a recipe for marshmallows.
But first a bit of history for those who know of marshmallows only as a manufactured food. Marshmallow is the name of a plant, native to Europe and Asia. It’s leaves are edible, but it is prized for the roots, which are ground and cooked and exude a gelatinous substance that formed the basis of the marshmallows we all know and some love. Nowadays, the marshmallow (gooey root extract) has been replaced with gum arabic (a gooey extract of the acacia senegal family of trees). The rest of the process has remained fairly consistent.
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.
Another method involves cooking the sugar syrup to the hard-ball stage, adding it to the gelatin, adding the flavoring, beating it for ages, then beating it into beaten egg whites, but since that introduces the whole uncooked-egg thing, we’ll skip it.