• Heat the oven to 350 degrees F and spray a 9 x 13-inch pan with cooking spray.
• In a large mixing bowl, combine the flour, baking powder, baking soda, cinnamon and salt.
• Separate the egg into 2 bowls. Add the lemon zest to the sugar and mix until the zest is well combined. Beat the egg whites and gradually add the sugar. Beat until stiff peaks form.
• Add the milk and vanilla to the yolks and stir well.
• Make a hole in the middle of the flour, then pour the yolk mixture into the flour and stir just until combined, making sure not to overmix.
• Fold in the egg whites gently until the batter is just combined.
• Spread evenly in the prepared pan.
• Add the chopped rhubarb to the top of the cake.
• Sprinkle the sugar over the cake, then gently pour the cream over the cake.
• Bake for 45 minutes, or until a toothpick comes out clean. The cake will form a custard over the bottom.

Make the Glaze:

• While the cake is baking, add the lemon juice and sugar to a small pan and bring to a low boil. Cook for 1 minute.
• Cool until just warm, then brush over the cake as soon as it comes out of the oven.