1. Spray a baking sheet with vegetable spray, wiping off any excess with a paper towel; set aside. In a large heavy-bottomed saucepan, combine sugar, corn syrup, and water. Stir to combine. Cover, and bring to a boil over high heat. Cook until sugar is dissolved. Uncover, reduce heat to medium, and continue to cook without stirring until the sugar mixture reaches 295 degrees.on a candy thermometer. Remove from heat.

2. Stir in 3 cups pistachios, the butter, vanilla, and baking soda. Return to heat, and cook, stirring constantly, until the mixture reaches 300 degrees.on a candy thermometer. Remove from heat, and pour mixture onto prepared baking sheet. Working quickly, spread mixture until about 1/4 inch -thick. Set aside to cool.

3. In a small bowl, combine praline dust and chopped nuts; set aside. Melt chocolate in the top of a double boiler set over a pot of simmering water. Pour one-half of the chocolate over the brittle, spreading with an offset spatula to cover completely. Sprinkle with half of the praline dust mixture. Transfer to refrigerator until set, about 15 minutes. Remove from refrigerator. Turn and coat other side with chocolate and sprinkle with remaining praline dust mixture. Chill until set, about 15 minutes. When chocolate is completely set, break into irregular pieces.