1. For the toffee: Adjust oven rack to middle position and heat oven to 250 degrees. Line bottom of 13 by 9-inch baking dish with waxed paper and spray paper with cooking spray. Arrange crackers, in single layer, salt side down, on waxed paper.
2. Combine sugar, corn syrup, and peanuts in medium microwave-safe bowl. Microwave on highest power until sugar melts, about 2 minutes. Stir and microwave until mixture begins to boil, about 2 more minutes. Stir in butter and vanilla until butter melts. Microwave until liquid begins to brown, about 2 minutes. Stir in baking powder.
3. Working quickly, pour sugar mixture over saltines and spread evenly (if mixture hardens too quickly to spread, place baking dish in oven for 5 minutes). Refrigerate until firm, about 1 hour. Invert brittle onto cutting board, peel away waxed paper, and, using hands, break into large pieces. Return pieces, toffee side up, to baking dish.
4. For the topping: Melt chips and butter until smooth. Stir in corn syrup and, using spoon, drizzle mixture over brittle. Let harden for 1 hour. (Brittle can be stored in airtight container at room temperature for up to 4 days.)
This recipe was submitted by one of our readers and has not been tested by our food editors. We are not responsible for errors in this recipe, but if you find one, please let us know.