In one bowl, combine Corn Flakes, butter, and brown sugar. Press into the bottom and up the sides of a 14 x 5 x 1 or 11 x 8 x 1 tart pan with a removable bottom. Bake for 8 to 10 minutes. Cool on a rack.
In a glass measuring cup, sprinkle gelatin over water to soften. Let stand several minutes. Microwave on high or until gelatin is dissolved. Stir.
In a large bowl, whisk together yogurt, milk, honey and vanilla. Whisk in dissolved gelatin. Pour into prepared crust. Chill 4 to 24 hours.
Peel oranges and slice into thin rounds. Discard any seeds. Remove tart from pan. Arrange orange slices on top. In a small saucepan heat marmalade just until melted. Brush over oranges and drizzle on top of tart.