Combine cocoa, sugar, corn syrup and salt in a heavy 3-quart saucepan.
Add evaporated milk and water; bring to a boil over medium heat, stirring
constantly. Cook, stirring frequently, until mixture forms a firm ball in cold
water, or registers 245 degrees F on a candy thermometer.
Remove from heat; drop in butter and vanilla extract, stirring to completely
blend in butter.