Melt butter over low heat. Remove from heat.
Stir in sugar and put backlow heat until mixture comes to a full boil,
stirring constantly. Stir in water and corn syrup.
Cook, stirring occasionally, to soft crack stage (270°).
Remove from heat. Stir in vanilla, then popcorn.
Mix lightly. Spread quickly in buttered 15x10" sheet pan.
Press down with spatula. Cool until firm.
Then break in large pieces.