In a 2-quart microwave-safe bowl (preferably a 2 quart glass measure with a handle), stir together the nuts, sugar and corn syrup. Microwave, uncovered, until the mixture is light brown, about 6 to 8 minutes, stirring well halfway through. Using oven mitts, remove the nut mixture from the oven. (It will be very hot.) Stir in the teaspoon of butter and vanilla. Continue to microwave, at high, 1 1/2 to 2 more minutes. The mixture should become caramel-colored. Meanwhile, grease a cookie sheet well with butter and set aside. Use oven mitts to remove the mixture from the microwave. Add the baking soda. Stir quickly until light and foamy and the baking soda is thoroughly mixed in. Immediately (and very carefully) pour onto the prepared cookie sheet and use the back of a spoon to spread out as quickly as possible. (The brittle will be fairly thick.) Let the brittle stand until cold, about 30 minutes. Break into pieces and serve. Store leftovers in an airtight container, preferably a tin, up to 2 weeks, depending on the humidity. NOTE: If you have whole cashews, break them into halves before starting. Calories: 177 per piece (40% from fat) Total fat: 8 grams Cholesterol: 1 mg Saturated fat: 2 grams