Place the edamame, soy sauce, cayenne pepper and salt into a small mixing bowl and stir to combine.
Line a half sheet pan with a silicone baking mat.
Place a 3-quart saucier inside a large cast iron skillet. Add the sugar and water to the saucier, and cook over high heat, stirring occasionally with a wooden spoon, until it comes to a boil. Stop stirring, cover, and cook for 3 minutes. Uncover, reduce heat to medium, and cook until the sugar is a light amber color, approximately 25 minutes. Remove from the heat and stir in the edamame mixture. Working quickly, pour the mixture onto the prepared half sheet pan and spread thin with an oiled spatula. You will have to work quickly when pouring out and spreading the mixture in the pan. Cool completely, approximately 30 minutes, and then break into pieces. Store in an airtight container for up to 2 weeks.