Line two baking sheets with parchment or wax paper. Place the egg whites in a clean, grease-free stand mixer bowl.

If using nuts, toast them whole at 350°F for about 5 minutes, then cool and chop them.

In a heavy saucepan, combine granulated sugar, brown sugar, light corn syrup, and water. Cook over medium-low heat, stirring until the sugar dissolves.

Once the syrup begins to boil, stop stirring and clip on a candy thermometer. Cook until the syrup reaches 255°F.

At about 240°F, add salt to the egg whites and whip on high until stiff peaks form.

When syrup reaches 255°F, remove it from heat and gradually drizzle into the whipped egg whites while the mixer runs on high.

Continue beating the mixture until thick, glossy, and able to hold its shape on the whisk.

Fold in the vanilla and, if using, chopped toasted nuts. Work quickly as the mixture begins to set.

Using buttered spoons, drop about 1 1/2 tablespoon portions onto the lined trays and let dry uncovered for 3 hours or until firm.