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Preheat the oven to 250 F.
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Make the popcorn, and spread it out in a large roasting pan. Scatter the pistachios over the popcorn--leave it scattered on top. I shook mine down and they did not stick to the popcorn as well because they ended up on the bottom, under the caramel.
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If you have a candy thermometer, place it beside your stove.
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Melt the butter in a medium heavy-bottomed saucepan over medium heat. Stir in the garam masala, and keep stirring for 1 minute. It should become quite aromatic.
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Add the brown sugar and light corn syrup. Stir once and then let come to a boil.
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If you have a candy thermometer, place it in the saucepan now.
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Stir the syrup while it boils, for about 3-4 minutes. It should reach 300 F.
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Remove from the heat and stir in the baking soda.
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Pour the caramel over the popcorn and toss gently to combine. Do not worry about whether it coats every piece evenly. Sprinkle with salt and place it in the oven.
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Bake for 1 hour, tossing the popcorn every 15 minutes (you will need to remove it from the oven to do this). If you want more salt, this is a good time to sprinkle more salt (if you are unsure, remove a piece and let it cool before trying it).
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When you remove the popcorn from the oven for the last time, toss again to prevent excessive sticking. Let cool completely before serving or storing.