Spray a parchment-lined sheet tray with nonstick spray.
Add butter to a large skillet. Add pumpkin seeds and stir to toast. Sprinkle with salt, cayenne pepper and cinnamon. Remove to a bowl.
In a saucepan, combine sugar, water and corn syrup, and stir until combined. Cook over medium heat until lightly golden amber and a candy thermometer reaches 290 degrees.
Stir in the pumpkin seeds and cook, while stirring, until pale golden brown and thermometer reaches 305 degrees, another 3 minutes. Stir in baking soda then quickly pour onto prepared sheet pan. Spread evenly with a rubber spatula. Cool and break into pieces.