Butter the sides of a large heavy saucepan. Add sugar, corn syrup and water;
bring to a boil, stirring constantly. Cook and stir over medium-low heat until
a candy thermometer reads 260 degrees F (hardball stage).
Stir in sunflower seeds and butter. Cook over medium heat to 300 degrees
F (hard-crack stage).
Remove from the heat; vigorously stir in vanilla extract and baking soda.
Pour into a buttered 15 x 10 x 1-inch baking pan; spread evenly to fill
Break into pieces.
Store in an airtight container with wax paper between layers.