Heat oven to 200°F. Line a half sheet pan with a silicone baking mat.
Take the pistachios and 35 grams (2 1/2 tablespoons) of the sugar for a spin around your food processor, until the mixture appears damp, thick, and begins to clump up, 1 to 2 minutes. (You’ll need to stop two to three times to scrape the sides and bottom of the bowl with a rubber or silicone spatula.) Add the cornflakes and pulse until crushed and the mixture is fairly uniform, 5 to 10 pulses. Transfer to a bowl and set aside.
Combine the water, corn syrup, and vanilla in a small (2-quart) saucepan. Place on the stovetop, then pour the remaining 225 grams sugar into the center of the pot so that it forms a mound. Turn the heat to medium until the mixture reaches a rolling boil. Swirl the pot occasionally to help the sugar dissolve completely, 5 to 8 minutes.
Continue boiling the syrup, without stirring, until the mixture registers 275°F, 8 to 12 minutes. Reduce the heat to low and continue cooking until syrup reaches 285°F, another 30 to 60 seconds. Immediately remove from the heat and carefully pour out in a rectangle in the center of the prepared sheet pan.
Tilt the sheet pan, working the syrup into a rectangle measuring roughly 12 by 9 inches. Turn the sheet pan so that the short side is facing you, then evenly spread the pistachio mixture over the bottom two-thirds of the candy. Using the mat, fold the candy as you would a business letter, folding the empty third over the middle, then folding that over the bottom third to create three layers of candy and two of pistachio filling. (If the mixture sticks to the mat, let sit for 2 to 3 minutes longer to continue to cool; it will release when it’s ready. You can also use a plastic dough scraper to help.) Fold the packet in half widthwise to form a 5-by-4 1/2-inch rectangle, then move to the oven for 5 minutes.
Remove the mat, with the candy still on it, from the sheet pan and on to the counter. Use a rolling pin to roll it back out to a 12-by-9-inch rectangle. It will take some force and patience and it won’t necessarily be pretty, but, again, you should not hear the candy cracking. (You may see some of the pistachio mixture coming through the sugar mixture, but that is okay; continue with the recipe.) If you do hear cracking, return the candy, on the mat, to the sheet pan and place in the oven to heat up for another few minutes. Once the candy is rolled out, repeat the folding process from step 5. Return the mat and candy to the sheet pan, and then back in the oven for 3 to 5 minutes.
Repeat step 7 once more, then return the candy to the oven to soften one last time.
Now use the rolling pin to roll the candy about 1/4-inch thick. Transfer the candy (not the mat) to a piece of parchment paper and place on cutting board. Trim the edges, if desired, then use a chef’s knife to slice into 24 to 32 fun-sized candies.
Wipe off the silicone baking mat and return it to the sheet pan. Place next to the board with the candies. Place a folded kitchen towel right next to the stovetop. Place a second folded kitchen towel in the bottom of a high-sided sauté pan or large skillet and add about 1 inch of water. Turn the heat under the sauté pan on to medium-high and let the water come to a bare simmer, then reduce the heat as low as it will go.
Place the chocolate in a metal bowl that will fit in the sauté pan, and set in the hot water. Stir constantly for 5 seconds. Remove from the heat, set on the towel, and stir vigorously for 30 seconds. Return to the hot water and stir for another 5 seconds, then back to the dry towel for 30 seconds. Repeat until the chocolate is half-melted, then reduce the time the bowl is on the heat from 5 to 3 seconds. Keep the off-heat stir time at 30 seconds. The goal is to melt all of the chocolate while keeping the temperature below 88°F. This process can take anywhere from 10 to 20 minutes, depending on the size of the chocolate pieces. If the chocolate begins to creep up above 88°F, leave the bowl on the counter and stir like crazy until the temperature drops back down. Continue with the melting process.
Once the chocolate is a smooth liquid, stir vigorously for another minute before dipping. Place a piece of candy into the bowl of chocolate and, using a fork, flip it over to coat both sides. Pull the candy from the chocolate, then tap the fork on the side of the bowl to remove as much excess chocolate as possible. Transfer to the silicone mat and repeat with the remaining candies. (If the chocolate becomes too thick or hard to work with, return it to the hot water for a few seconds, stirring constantly. Again, do not let the temperature rise above88°F.)
Rest the candies at room temperature until the chocolate hardens, up to 1 hour. Devour or store in an airtight container at room temperature for up to 2 months.