Heat the oven to 200 degrees F to keep the baking sheet warm.
Combine the sugar, 1 cup of water, and the corn syrup in a heavy-bottomed saucepan.
Cook over medium heat, stirring occasionally until a candy thermometer reaches 240 degrees F (soft ball stage), about 25 minutes **see notes
While the caramel is cooking, place the baking sheet in the oven to keep it warm.
Remove the caramel from the heat and stir in the butter and cashews.
Return to the heat and cook until the thermometer reaches 300 degrees F (hard crack stage).
Whisk together the baking soda, water, and vanilla extract in a small bowl.
Remove the brittle from the heat and immediately stir in the baking soda mixture until light and foamy.
Remove the warm pan from the oven and generously coat it with butter.
Pour the candy mixture onto the prepared baking sheet and spread it quickly to ¼ inch thick with a large buttered offset spatula. We used a 12x18 baking sheet and the brittle fit perfectly to the edge.
Allow the brittle to cool completely.
Break the brittle into pieces and store it in an airtight container at room temperature.