For the Pudding: In a small bowl, whisk together 1/2 cup milk, 1/3 cup sugar, eggs, egg yolk and cornstarch. In a medium saucepan, bring 2 cups milk and 3 tablespoons sugar almost to a boil. Whisk in other ingredients. Bring to a low boil. Whisk constantly until thickened and smooth, about 2 minutes. Remove pan from heat. Strain custard and whisk in vanilla. Place 2 bananas in food processor and pulse until cut into small pieces. Fold this into custard and refrigerate.
For the Napoleon Layers: Preheat oven to 350 degrees F. Whisk egg whites with sugar. Stir in melted butter and vanilla extract. Fold in flour. On a parchment lined sheet pan, spread 1 tablespoon of mixture thinly into a 4-inch diameter circle (like a thin cookie). Repeat until there is no more batter. Bake in the oven until light brown, about 5 minutes. For each serving, use 3 "cookies". Slice thin remaining bananas on bias. On each plate, place 1 cookie, followed by 2 slices of bananas, 1 1/2 tablespoons of banana custard. Repeat and then top with remaining cookie. Garnish with powdered sugar and mint leaves.