Air-pop the kernels for the cleanest result, or heat 1–2 tsp oil in a heavy pot over medium-high. Test with 3 kernels, then add the rest in an even layer, cover and shake until popping slows. Transfer all popped corn to a very large bowl, remove unpopped kernels, and sprinkle 1 tsp salt if using. Ensure popcorn is dry and cool.

Preheat oven to 250°F (120°C). Line two rimmed baking sheets with parchment or silicone mats.

In a heavy saucepan combine butter, brown sugar, corn syrup and water (if using). Heat over medium, stirring until sugar dissolves and the mixture comes to a steady boil. Stop stirring and attach a candy thermometer.

Boil to 250°F (121°C) (firm-ball stage). This usually takes 4–7 minutes after a rolling boil. Watch closely to prevent boiling over or burning.

Remove from heat and immediately stir in baking soda and vanilla — the mixture will foam and lighten in color.

Quickly pour foamy caramel evenly over the popped corn. Use a silicone spatula to fold gently and coat without crushing. Spread coated popcorn in an even layer across the prepared baking sheets (two sheets rather than a deep pile).

Bake at 250°F (120°C) for 45–60 minutes, stirring and turning every 15 minutes to ensure even drying and a brittle, crisp coating. If caramel still feels wet after 45 minutes, bake up to 60 minutes.

Remove sheets and cool completely (20–30 minutes). Once cool and set, break into clusters. Gently fold in the chopped Snickers so pieces remain distinct (adding them after cooling prevents melting).

Melt chocolate gently (microwave in 20-second bursts or double boiler) and drizzle over the popcorn. Sprinkle flaky sea salt to taste. Allow chocolate to set briefly before serving.