Lightly grease a large baking dish (approx. 14 x 10 inches).

In a large pot or Dutch oven, heat the vegetable oil over medium heat. Add 2–3 popcorn kernels and cover. When they pop, remove them, then add the rest of the kernels in an even layer. Cover and cook, shaking the pot occasionally, until popping slows to about 1 pop every 2–3 seconds. Remove from heat and transfer popcorn to a very large bowl, discarding any unpopped kernels.

Wipe out the pot, then melt the vegan butter over low heat. Stir in brown sugar, syrup, salt, and vanilla. Once it begins to bubble, reduce the heat to the lowest setting and let it simmer without stirring for 3 minutes—watch closely, as it can burn easily.

Remove from heat and immediately whisk in the baking soda (it will foam slightly). Quickly pour the caramel over the popcorn and stir until evenly coated.

Press the coated popcorn into the greased baking dish, using a spatula to flatten it gently and evenly.

Drizzle the melted dark chocolate evenly over the popcorn. Sprinkle with sea salt flakes.

Refrigerate for about 30 minutes or until the chocolate is set. Use a hot knife to cut into slices or bars.