Line two baking sheets with parchment or wax paper. Place the egg whites in a clean, grease-free mixer bowl. If using nuts, toast them at 350°F for about 5 minutes, then cool and chop.

In a saucepan, combine granulated sugar, brown sugar, light corn syrup, and water. Cook over medium-low heat, stirring until the sugar dissolves. Wipe the sides of the pan with a damp pastry brush.

Once the syrup begins to boil, stop stirring and clip on a candy thermometer. Cook until the syrup reaches 255°F (hard-ball stage).

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

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

Beat the mixture until thick and glossy, strong enough to hold its shape.

Fold in the vanilla extract and nuts (if using). Work quickly as the mixture begins to set.

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