Chop the bacon and cook until crisp. Drain on paper towels and set aside.
Preheat the oven to 250F. Line a large baking sheet with foil and butter the foil. Place in the oven while you make the brittle.
In a large saucepan (at least 2 quarts), stir together the sugar, brown sugar, corn syrup and water. Bring to a boil. Place a candy thermometer into the mixture and cook, stirring frequently, until the temperature reaches 290F. Stir bacon pieces and the cashews and continue to cook until the mixture reaches 300F.
Remove the pan from the heat and stir in the butter and baking soda. (It will foam up - be careful!) Stir just until combined, then remove the prepared pan from the oven and pour the mixture onto the pan. Use 2 forks to lift and pull the brittle out until thin.
Let cool completely before breaking into pieces and serving.