Add popcorn kernels to paper bag, fold over top twice, and microwave on high for 2 1/2 to 3 minutes. Stop the microwave when the popping slows to one to two pops per second. Add popcorn to a large bowl.
Melt the butter in a large heavy-bottomed pot over medium-high heat. Continue cooking until butter is no longer foaming and turns a medium brown (tilt the pan occasionally to check color). Reduce heat to medium and stir in salt, brown sugar and cayenne pepper.
Pour butter over popcorn and toss to coat. Sprinkle with additional salt, to taste.
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