Preheat the oven to 325°. Spread the cashews on a small baking sheet and toast for 10 minutes, stirring once. Keep the cashews warm.
Line a rimmed baking sheet with buttered parchment paper. In a saucepan, bring the sugar, corn syrup, butter and water to a boil. Cook over moderately high heat, stirring, until a deep amber toffee forms, about 10 minutes. Remove from the heat. Whisk in the baking soda and cayenne; it will bubble up. Using a wooden spoon, stir in the warm cashews.
Pour the hot candy onto the baking sheet and spread it as thinly as possible. Let cool completely, then break into large shards.
Line a baking sheet with parchment paper. In a heatproof bowl set over (not in) a saucepan of simmering water, melt two-thirds of the chocolate. Stir until the chocolate reaches 130°. Remove the bowl from the heat and stir in the remaining chocolate until melted and the temperature drops to 85°. Return the bowl to the saucepan and warm the chocolate to 90°, stirring. Remove from the heat. Fold in the Rice Krispies. Spread the mixture on the parchment in a 1/4-inch-thick layer. Let harden completely, then break into large shards.
Milk Chocolate Bark: Use milk chocolate instead of dark. Follow the same method: Melt two-thirds of the chocolate to 113°. Add the remaining chocolate and stir until 80°. Warm the chocolate to 85°; fold in the Rice Krispies.