Butter sides of a heavy 2-quart saucepan. In pan melt butter. Add sugar, water, and corn syrup. Cook and stir over medium-high heat until mixture boils. Clip a candy thermometer to side of pan. Reduce heat to medium; continue boiling at a moderate, steady rate, stirring frequently, until thermometer registers 290 degrees F (soft-crack stage), about 15 minutes. Watch carefully after 280 degrees F to prevent scorching.
Remove saucepan from heat; remove thermometer. Quickly stir in toasted pumpkin seeds. Pour candy into prepared pan; spread evenly (mixture will be thick). Let candy stand until firm. When candy is cool and firm, use foil to lift from pan. Break into pieces. Store tightly covered.