- Generously butter an 11 x 8-inch (28 x 20 cm) baking dish.
- In a high-sided pot, bring the sugar, corn syrup, honey, water and salt to a boil. Cook without stirring until a thermometer reads 252°F (122°C).
- In a stand mixer fitted with the whisk attachment, beat the egg whites until stiff peaks form. With the machine running, drizzle 1/3 of the hot syrup over the egg whites, avoiding the whisk. Beat constantly until the mixture thickens, about 4 minutes.
- Meanwhile, return the remaining syrup to the heat and continue cooking, without stirring, until a thermometer reads 315°F (157°C). With the machine running, drizzle all of the hot syrup over the egg whites, avoiding the whisk. Beat constantly until stiff peaks form again, about 10 minutes.
- For this step, you need to work fast because the mixture will harden quickly. With the mixer on low, beat in the butter and vanilla. Using a spatula, fold in the pistachios. Spread the mixture evenly in the prepared baking dish, pressing it down firmly with lightly oiled fingers. Let set at room temperature until firm, about 2 hours. Unmould.
- On a work surface, cut the nougat into squares with an oiled knife. Wrap each square in wax paper. The pistachio nougat will keep at room temperature for 3 weeks.