Summer in Taiwan is only bearable with this: a caramelly, sweet, custardy, cold sweet potato. That's right: The sweet potatoes here are exactly as it sounds — baked then frozen. In Taiwan, these are so commonplace, you'll find them packaged in plastic bags at convenience stores. But oddly enough, they are some of the best sweet potatoes you've ever had.
Technique tips: Depending on the size of your oven, you can bake as many as 12 sweet potatoes at once. For small toaster ovens, I recommend 3 to 5 sweet potatoes at once. You want to have enough space between each sweet potato for hot air to circulate.
Different varieties of sweet potato contain different amounts of sugar content — choosing the correct one is key. For this recipe, I recommend using Taiwanese, Japanese or Korean sweet potatoes. I've tried many sweet potato varieties and found the best results with Japanese sweet potatoes (the reddish color skin, pale-yellow flesh variety also known as satsumaimo). Sugar content is pivotal so do not use starchy varieties like the Okinawan sweet potato (purple flesh). The ideal shape will be long, slender sweet potatoes that are not too round or too big. The ideal weight will be 10 to 14 ounces. They also should not be sprouting, as that process tends to reduce sugar content.
Preheat the oven to 450 F with convection on, if available. You may also want to place a sheet pan on the rack below to catch any sugar drippings. Brush and wash the sweet potatoes very well, then dry completely.2.
Place sweet potatoes directly on the oven rack and let bake. The baking process will take 40-80 minutes, depending on the size and shape of your sweet potatoes. The way to tell when they're done is by poking and tapping the skin: it should feel hardened and almost completely separate from the flesh, creating a hollow sound. The kitchen should smell distinctly of dark caramel. Once the sweet potatoes are at your desired doneness, take them out of the oven and let them rest on a wire rack. (Note: You can enjoy some hot and freeze the rest.)3.
Once the sweet potatoes have cooled completely, pack them in a freezer-safe bag and freeze for about 4 hours. This will get you the perfect ice cream-like texture. (Note: Letting the baked sweet potatoes cool completely prevents condensation in the freezer bags.) Alternatively, you can freeze them uncovered without cooling, then pack them in bags later. If freezing overnight, let the sweet potatoes thaw for about 10 minutes at room temperature before enjoying. Alternatively, microwave the sweet potato for about 30 seconds, flipping halfway to ensure even heating. Eat it while cold!