Preheat your oven to 250°F (120°C). Line two large rimmed baking sheets with parchment paper or silicone baking mats. Pop your popcorn and measure out 10 cups into a very large mixing bowl. Remove and discard any unpopped kernels.

If using nuts or pretzels, add 1–1 1/2 cups of nuts and/or 1–1 1/2 cups of mini pretzels to the bowl with the popcorn. Toss gently to combine, keeping everything in one large, roomy bowl.

In a medium saucepan over medium heat, melt the butter. Stir in the brown sugar, corn syrup, and salt. Bring to a gentle boil, stirring often until the sugar dissolves. Once bubbling, stop stirring and let cook for 4–5 minutes, until the caramel is slightly thickened, glossy, and a deep golden color (similar to maple syrup).

Remove the saucepan from the heat. Carefully stir in the vanilla extract—it will bubble up. Stir in the baking soda; the caramel will foam and lighten in color, which helps it coat the popcorn evenly.

Immediately pour the hot caramel over the popcorn (and any mix-ins) in the large bowl. Using a large spatula or wooden spoon, gently toss and fold until the popcorn is as evenly coated as possible. It’s fine if some spots look patchy; baking will help spread the caramel.

Spread the caramel-coated popcorn onto the prepared baking sheets in an even layer (it doesn’t have to be a single layer, but avoid deep piles). Bake at 250°F (120°C) for 45–60 minutes, stirring every 15 minutes. Each time you stir, break up clumps and rotate the popcorn from the edges toward the center. The caramel corn is done when it feels mostly dry to the touch and smells toasty and buttery.

Remove the pans from the oven and let the caramel corn cool completely on the baking sheets. While it is still warm but safe to touch, break apart any larger clusters. Allow it to cool until fully crisp and dry.

In a microwave-safe bowl, combine the chocolate chips and neutral oil. Microwave in 20–30 second bursts, stirring after each interval, until melted and smooth (about 60–90 seconds total). Alternatively, melt the chocolate in a heatproof bowl set over a pan of barely simmering water.

Once the caramel corn is cool, gently bunch it a bit closer together on the baking sheets (not piled too deeply). Using a spoon or a piping bag/zip-top bag with a snipped corner, drizzle the melted chocolate back and forth over the popcorn. If using sprinkles, add them right away while the chocolate is still wet. If using additional melted white chocolate, drizzle it on at this stage as well.

Let the chocolate set at room temperature for 30–45 minutes, or chill the pans in the refrigerator for 10–15 minutes to speed it up. Once the chocolate is firm, break the caramel corn into clusters and serve, or transfer to airtight containers or bags for storage or gifting.