In a small saucepan over medium heat, bring the agave nectar to a boil, stirring frequently. Decrease the heat to low and simmer for 4 to 8 minutes, watching constantly and stirring occasionally, until the agave nectar darkens slightly from its original amber color. In a large bowl, using a handheld mixer, whip the egg whites to stiff peaks. Drizzle the agave nectar slowly into the egg whites, whisking continuously until blended. Use to frost cakes or cupcakes, or as a filling between cookies. Store in a glass Mason jar in the refrigerator for up to 24 hours.