Do not prepare this frosting ahead as it will harden very quickly.
Bring first 3 ingredients to a boil in a 2-quart saucepan over medium heat, stirring often, and boil 1 minute. Remove from heat, and whisk in 1 cup powdered sugar and vanilla extract until smooth. Stir in toasted pecans, stirring gently 3 to 5 minutes or until mixture begins to cool and thicken slightly. Pour immediately over cake.