AllRecipes.com has a highly-rated recipe for cashew brittle called "Nat's Buttery Cashew Crunch." It is very similar to brittle you might find at an Amish store and requires only five ingredients and a candy thermometer.
For this recipe, you need 1 cup unsalted butter, 1 1/4 cups granulated sugar, 1 tablespoon light corn syrup, 1/4 teaspoon salt and 10 ounces of cashews. Start by greasing a large baking sheet. Combine butter, sugar, salt and corn syrup in a saucepan. Cook over medium heat, stirring constantly until butter is melted. Add the cashews, stir, and cook over high heat until the mixture reaches 300 to 310 degrees F. With a wooden spoon, stir occasionally to keep the cashews from burning. Once the mixture reaches the proper temperature, pour it onto the greased baking sheet. Allow to cool for 10 minutes. Once cooled and hardened, break the candy into pieces. Store it in a waxed paper-lined, airtight container.
Total preparation time is approximately 20 minutes. Other nuts may be substituted in place of cashews. Candy cooked to 300 to 310 degrees F is at the hard-crack stage, which means almost all water has been evaporated. To test whether your candy has reached this stage, add a small drop of the molten candy into cold water. It should form hard, brittle threads when taken out and pulled apart. Always use caution with candy at this temperature, and only allow children to prepare under adult supervision.