I have a peanut allergy and absolutely loved this cashew brittle. You'll want to make it again and again. Spread 1 1/2 tablespoons butter all over a rimmed baking sheet until heavily coated. Stir sugar and corn syrup together in a microwave-safe bowl. Microwave on high, uncovered, for 4 1/2 minutes. Stir and microwave for 3 minutes longer.
In 1 1/2 qt casserole, stir together sugar and syrup. Micro on high for 4 minutes. Stir in cashews. Micro on high 3-5 minutes only until light brown. Add butter and vanilla, blending well. Micro on high 1-2 minutes more. Nuts will be lightly browned and syrup very hot! Add baking soda and stir gently stir until light and foamy.
For traditional peanut brittle, substitute 3 cups unsalted raw Spanish peanuts for the cashews. I made this as written and found it wonderful! My friends said it was the best brittle they have every had! I used both my thermometer and ice water to double check and found the 14 minutes to be spot on!