Melt butter in a small saucepan over low heat for 1-2 minutes, stirring occasionally until fully liquid but not browned; remove from heat immediately.
Toast dry spices (garlic powder, paprika, cayenne pepper) in a dry skillet over medium heat for 1 minute, stirring constantly until fragrant but not smoking.
Toss freshly popped popcorn in a large bowl with the melted butter, using a spoon or hands to coat kernels lightly and evenly; work quickly before the butter cools.
Sprinkle the seasoning blend over the buttered popcorn, then toss again for 20-30 seconds until every kernel is evenly coated; add salt last and taste before adding more.
Serve immediately or cool completely on a baking sheet before storing in an airtight container to avoid sogginess.
