Line a baking sheet with parchment paper and butter the paper. In a medium saucepan, combine the sugar, corn syrup and water and bring to a boil over moderately high heat, stirring just until the sugar dissolves. Continue to cook, undisturbed, until a deep amber caramel forms, 8 to 9 minutes. Remove from the heat. Add the pumpkin seeds and the dissolved baking soda and stir until combined, then quickly pour the mixture onto the baking sheet, spreading it into a thin layer with a spatula. Let cool until hardened. Break into shards and serve.