Step 1Place one-half cup of the water in the bowl of a stand mixer fitted with the whisk attachment, or, if using a hand mixer, in a large bowl. Sprinkle the gelatin over the water and set aside.
Step 2In a medium, heavy-bottom pan, add the remaining one-half cup water and stir in the sugar and corn syrup. Bring to a boil over high heat. Cook the mixture, swirling the pan occasionally, until the syrup reaches 240 degrees on a candy thermometer (the mixture will be frothy and bubbling vigorously). Meanwhile, butter a 12-by-17-inch rimmed baking sheet. Line with parchment and butter the parchment too.
Step 3Whisk the gelatin mixture on low speed and add the 240-degree syrup down the side of the bowl in a steady, slow stream. Gradually raise the speed to high and whisk until the mixture is fluffy and glossy and has tripled in volume, about 12 minutes. Add the salt, vanilla and cinnamon and whisk until thoroughly combined.
Step 4Using a lightly greased spatula, immediately spread the mixture out onto the baking sheet, smoothing it so an even layer completely covers the pan. Let stand, uncovered, at least 4 hours and preferably overnight to set.
Step 5When the marshmallows have set, dust the top with a light coat of powdered sugar. Coat small cookie cutters with powdered sugar and cut out as many as you can from the sheet. Dust the marshmallows with a little more sugar so that they don't stick together. Store in a sealed container for up to one week.