Sprinkle the powdered gelatin over the cold water. Set aside.
Place the egg yolks, ½ cup granulated sugar, orange juice and orange zest in a heat-proof bowl. Whisk until combined.
Place bowl over a double boiler with simmering water (note: bottom of bowl should not touch the water). Cook over simmering water whisking often. Custard will begin to thicken and will reach 180° F when done.
Add set gelatin to the bowl and stir until gelatin melts.
Remove bowl from simmering water and allow custard to cool to room temperature.
Begin whipping egg whites until they are frothy. Slowly pour ¼ cup of granulated sugar into the egg whites while continuing to mix. Whip until egg whites reach medium peaks.
Gently fold the egg whites into the orange custard until no white streaks remain.
Pour the filling into the prebaked crust. Place in refrigerator until set (approximately 2 - 3 hours).