Step 1Line a baking sheet with Silpat or parchment paper.
Step 2Make the honeycomb: Clip a candy thermometer to the side of a medium, heavy-bottom saucepan. Combine the sugar, corn syrup and water in the pan and bring the mixture to a boil over medium-high heat, using a pastry brush dipped in water to brush down the sides of the pan and swirling the pan occasionally for even browning, as the sugar cooks. Cook the sugar to hard crack stage, or until the thermometer registers 300 degrees; it will be a very pale straw color. Working quickly, add the baking soda and whisk for a few seconds to incorporate it into the caramel; it will bubble up and become foamy. Pour the foamy mixture onto the prepared baking sheet. (The honeycomb should spread out and puff up slightly and have a porous, shiny surface. If it spreads so thin that it flattens out like a pancake, this means either that you didn't cook the sugar long enough or that the baking soda was old. If puffs of smoke come through the pores, it means that the candy has burned. In either case, dump it and start again.) Set the baking sheet aside to cool the honeycomb to room temperature, about 20 minutes. Proceed with the dipping instructions or cover the honeycomb with plastic wrap until you are ready to dip it in chocolate.
Step 3When you're ready to dip the honeycomb, line a baking sheet with parchment paper or Silpat. Place the block of honeycomb on a cutting board and use a small paring knife to cut it into uneven chunks roughly 1 1/2-inches square. Discard the small shards and place the honeycomb pieces on one half of the prepared baking sheet.
Step 4To dip the honeycomb, have two dinner forks in front of you. Place one piece of honeycomb in the tempered chocolate to coat it. Lift the honeycomb out of the chocolate with one fork and tap it against the other fork to drain off as much chocolate as possible. Set the chocolate-covered honeycomb on the empty half of the prepared baking sheet and repeat with the remaining honeycomb pieces. When all of the honeycomb has been dipped in chocolate, sprinkle the pieces liberally with pistachios. Place the honeycomb in the refrigerator just until the chocolate hardens, 15 to 20 minutes. Transfer the honeycomb to an airtight container and refrigerate it for up to 2 days.