In a small bowl, mix together baking soda, 1 teaspoon of water and vanilla: Set aside.
In a 3-quart saucepan, add sugar, 1 cup water and corn syrup. Mix well.
Cook over medium heat, stirring occasionally to 240 degrees on a candy thermometer. It takes approx 25-30 minutes.
Stir in butter and pistachio nuts. Cook stirring constantly until the thermometer reaches 300 degrees. Watch carefully to make sure it doesn’t burn. This takes approx 10-15 minutes.
Remove from heat immediately and stir in baking soda mixture.
Working quickly, pour half the candy mixture on each prepared baking sheet and spread to a ¼-inch thickness.
Cool completely. Break into pieces.
Store cooled brittle in an airtight container. Pistachio brittle will keep for several weeks.