Notes about the recipe: I'm always searching for the perfect crunchy toffee that's not sticking to your teeth once you bite into it, and I think I have it here! I serve this on my elaborate petit-four cart as one of 12 treats at the end of the very long meal at my restaurant Tru. I have recently discovered (and become totally addicted to) the marvelous Marcona almond, the favorite of Spanish chefs and tapas-lovers. I nibble them with wine before dinner; I add them to salads; I serve them with cheese; and I stick them into every dessert I can — sometimes all in one day! Marconas are tender and toasty, never hard and dry like some supermarket almonds: you can buy them online at www.tienda.com. This combination of almonds with crunchy toffee and bittersweet chocolate is fantastic. You'll be amazed that you made it — and so will any friends that you give it to. It makes a great holiday gift.
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.