1. Preheat oven to 350 degrees.

2. Make the brittle: Bring sugar, corn syrup, water, and salt to a boil, stirring until sugar has dissolved. Wash down sides of pan with a wet pastry brush, and cook until pale amber, about 12 minutes total. Stir in butter. Remove from heat, and stir in cashews. Pour onto a parchment-lined baking sheet to cool.

3. In a food processor, finely grind the brittle. Place brittle in heaping teaspoons on a nonstick baking mat, and flatten each into a thin round. Bake until melted, about 5 minutes. Press a 1- to 2-inch heart-shape cutter into center of each round while brittle is still hot. With cutter still in place, pull away sides of melted brittle. Remove cutter. Let cool slightly, until bendable. Lift heart from baking mat with a spatula, and bend slightly at center of heart. Let cool completely.