Put the popcorn kernels and oil in a large, wide pot, stir to coat, cover and heat over medium heat. Once you begin to hear the kernels pop, shake the pot until the popping subsides, about 8 minutes. Remove the pot from the heat, and transfer the popcorn to a large bowl.
Bring a couple inches of water to a high simmer in a large saucepan. Set into the saucepan a bowl that fits snugly without touching the water. Add the chocolate, butter, vanilla and salt, and heat, stirring occasionally, until all the chocolate and butter melts. (The fat from the chocolate and butter will separate, and that's OK.) Remove the bowl from the saucepan, and vigorously whisk in the sugar. The mixture should come together and be thick and creamy.
Pour the chocolate coating over the popcorn, add the sprinkles, then use your hands to toss the popcorn and evenly coat it with the white chocolate. Spread the popcorn out onto a baking sheet to let the coating set up, about 30 minute. (The popcorn can be stored in an airtight container at room temperature overnight.)
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