Line a sided sheet pan with a silicone baking mat, or oil it well with vegetable oil (or use a heavyweight nonstick sheet pan).
Pour the sugar into the center of a saucepan fitted with a candy thermometer. Pour the water around the edge of the pan and wait to allow the water to moisten the sugar. (You can draw a clean finger through the center of the pan to allow some water to seep in.) Add the corn syrup and bring to a boil. Add the butter and boil until the mixture reaches 300 degrees F.
Turn off the heat and whisk in the cocoa; then stir in the nuts. Quickly pour the mixture onto the center of the prepared pan and let it spread out — it may not reach the sides of the pan. Set aside to cool at room temperature until hard. Using your hands (I wear gloves to avoid fingerprints), pry the toffee out of the pan and break into large pieces. Store in an airtight container. The toffee will keep well for up to 2 weeks in an airtight container.