Smoothly line 2 pans, 9 by 13 inches, each with a single piece of foil, crimping edges over pan rims to hold foil in place. Butter foil and set pans aside.
In a 5 to 6 quart pan over medium-high heat, cook 1 1/4 cups of butter, sugar, salt and water until mixture reaches 250 degrees on a candy thermometer, 6-10 minutes; stir often. Add almonds and hazelnuts; stir until mixture reaches 300 degrees, 4-8 minutes longer.
Remove from heat and stir in 3/4 cup of the pistachios. Protecting hands and working quickly, scrap candy equally into the 2 pans and spread evenly.
Let toffee cool until firm to the touch, about 2 minutes. Evenly sprinkle with the chocolate chips and let stand until the chocolate is softened, about 5 minutes. With a flexible spatula, spread the chocolate over the toffee. Chop the remaining pistachios and scatter onto the soft chocolate. Chill uncovered, until the chocolate is firm, about 15 minutes.
Lift foil with candy from the pans. Break toffee into chunks. Serve, chill airtight up to 2 weeks or freeze to store longer. Makes about 2 1/2 pounds.