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1
In a saucepan, combine the brown sugar, water, corn syrup, butter, and cream of tartar (this bit of acid keeps the sugar from crystallizing.)
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2
Bring to a simmer over medium heat, stirring frequently with a wooden spoon to dissolve the sugar.
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3
When the sugar syrup reaches a boil, stop stirring and swirl the pan around over the heat so it doesn't burn.
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4
As the syrup reaches the caramel stage, the bubbles on top will become smaller, thicker, and closer together.
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5
Simmer for 10 to 12 minutes, until the caramel coats the back of the spoon.
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6
(If so inclined, stick a candy thermometer in the pot and cook until the syrup reaches the soft-crack stage, about 270 degrees F. When you drop a bit of this syrup into cold water, it will solidify into threads that, when removed, are hard yet bendable.)
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7
Remove the caramel from the heat and stir in the marshmallows.
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8
Fold the marshmallows into the hot caramel so they all melt into a gooey caramel sauce.
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9
The caramel will coat better if the popcorn is hot, so make it while boiling the sugar or pop it first then keep it warm.
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10
Put the popcorn in a big bowl and pour the caramel marshmallow mixture over the popcorn.
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11
Gently fold with a wooden spoon to coat each kernel.
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12
Grab handfuls of the popcorn and squeeze it together to form popcorn balls; make them about the size of a tennis ball.
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13
Put the popcorn balls on a sheet pan lined with waxed paper to cool completely.
