Thaw one sheet of puff pastry until room temperature.
Preheat oven to 375 degrees.
Roll out into a 8" x 14" rectangle. Transfer to a parchment lined baking sheet.
Score edges of puff pastry creating a 1/2 inch border all around the pastry. You don't want to cut all the way through. Just make a score.
Brush pastry, inside of border, with beaten egg.
Peel and slice apples. Toss with lemon juice.
Melt butter in a mixing bowl. Mix with cinnamon, a dash of salt, and brown sugar. Add to apples and toss together.
Arrange apples as desired over puff pastry making sure to keep them inside scored border. Pour any remaining brown sugar mixture over apples.
Bake at 30-35 minutes or until apples are tender and pastry is golden brown.
Warm vanilla frosting in microwave until it reaches drizzling consistency. Drizzle over tart.
Serve warm or at room temperature.