Line two baking sheets with parchment paper.
Fit a large piping bag with a flower tip or star tip.
Place the piping bag in a tall glass with the top folded over.
Add apple puree, 1 cup sugar, vanilla, and egg white to a mixer bowl.
Beat on medium-high for 6 to 8 minutes, until thick and pale.
Add agar powder and water to a small saucepan.
Let the agar hydrate for 5 minutes before heating.
Stir in 1 1/2 cups sugar and corn syrup.
Bring the syrup to a steady boil over medium heat.
Cook until the syrup reaches 230°F.
With the mixer running, pour syrup slowly down the bowl side.
Beat 4 to 6 minutes, until glossy and very thick.
Add gel color during the final minute, if using.
Transfer mixture quickly into the piping bag.
Pipe flowers onto parchment while the mixture still looks smooth.
Let flowers dry uncovered for 8 to 12 hours.
Mix powdered sugar and cornstarch together.
Dust the flowers with the powdered sugar and cornstarch mixture before serving.