Prepare a baking sheet by lining it with a silicone mat or parchment paper, or grease it with extra butter. Set this aside.
In a large pot over high heat, combine the butter, sugar, and corn syrup. Stir this mixture frequently as it heats to prevent the sugar from burning.
Once the mixture has fully melted and starts to bubble, stir in the cashews. Keep stirring the mixture constantly until it reaches 310°F (154°C).
After reaching the required temperature, pour the hot candy mixture onto your prepared baking sheet. While the mixture is still hot and pliable, evenly sprinkle the sea salt over the top.
Allow the brittle to cool completely. When it's set, break the sheet of brittle into bite-sized pieces by gently dropping it or cracking it with a utensil.
Do not chill the candy or else it may "sweat." Wrap giftable amounts in parchment and place into a plastic box or tin with a lid. Serve and enjoy!
Notes
Adjust the amount of sea salt as necessary, depending on your preference for a saltier or less salty brittle.