Line an 8-in. (20-cm) square baking pan with parchment paper.
Melt butter in large saucepan over low heat. Add marshmallows; stir frequently until completely melted. Stir in vanilla. Remove from heat. Cool slightly, 2 to 3 min. (If the mixture is too hot, the popped popcorn will deflate.)
Working quickly, stir popcorn into marshmallow mixture until coated; fold in chocolate almonds. Scrape into prepared pan. With slightly dampened hands or buttered spatula, press popcorn mixture evenly in pan. Cool completely at room temperature. Cut into 16 squares.
Shape popcorn mixture into balls or clusters for variations.
Substitute dark chocolate covered almonds with milk chocolate covered almonds.