For cookie cutter shapes, use a 9 x 13-inch cookie sheet with a half-inch lip. For square cubes cut with a knife, use an 8 x 8-inch casserole dish.
Pour ½ cup water into the bowl of a stand mixer fitted with whisk attachment. Add gelatin and let sit to soften, about 10 minutes. Meanwhile, combine the sugar, evaporated milk, and ¼ cup water in a medium saucepan. Whisk constantly over low heat until sugar dissolves, then turn up to medium/medium-high. Bring to a gentle boil, then immediately reduce to medium-low. Don’t stir at all as the mixture continues to boil and turns a darker color. Wait patiently for a candy thermometer to reach 240 to 245 F (about 15 minutes.) Remove from heat immediately.
Pour the hot syrup into the gelatin mixture and beat on low until incorporated. Add vanilla extract and beat on high until thick, fluffy, and tripled in volume, about 10 to 15 minutes. While this is happening, lightly coat cookie sheet/baking dish, along with a few spatulas, with nonstick spray. Mixture is ready when it becomes glossy, thick, and white in color and when beater is lifted, the mixture slowly drips back into the bowl. Quickly scrape mixture into prepared pan with a spatula and smooth out top with the second spatula. Allow to set uncovered for at least 8 hours.
Combine powdered sugar and cornstarch. Sift some over the surface of the marshmallows. Use cookie cutters or a large knife to cut the marshmallows. Sift or roll cut marshmallows in powder to prevent sticking. Store in an airtight container for up to 2 weeks.
Per serving: 108 calories, 0 grams fat, 1 milligram cholesterol, 28 milligrams sodium, 12 grams total carbohydrates, 0 grams fiber, 11 grams protein.