Preheat the oven to 250°F.

Place popcorn in a large baking dish and scatter pistachios over it.

Melt butter in a medium saucepan over medium heat.

Add garam masala and cook, stirring, until aromatic (about 1 minute).

Add brown sugar and corn syrup and cook, stirring often, until the sugar has dissolved and the mixture comes to a boil.

Continue cooking for 3 to 4 minutes, stirring constantly.

Remove from heat and stir in baking soda; the mixture will lighten and expand.

Pour the caramel mixture over the popcorn and pistachios and stir to combine.

Sprinkle salt evenly over the popcorn.

Bake for 1 hour, stirring every 15 minutes to distribute caramel evenly.

Set aside to cool, stirring occasionally to avoid sticking.

When cool, transfer to a serving bowl or store in an airtight container.