Line a baking sheet with a lightly buttered piece of waxed paper, set aside. In a heavy medium sized saucepan, heat the butter, sugar and corn syrup on medium low heat. Stir occasionally and heat until temperature reaches 290 degrees on a candy thermometer (about 30 minutes). Take pan off heat and stir in cashews. Pour mixture out onto the waxed paper lined pan, and spread the candy out across the pan. Let it cool in the pan for 10 minutes, then in the fridge for 5 more minutes before peeling off the waxed paper. Break into bite sized pieces and store in an airtight container.