Squeeze lemon over sliced bananas. Place half of the banana slices in the bottom of the prepared pie crust. Set aside.
Combine corn starch, water and salt in a heavy-bottomed saucepan. Whiskwhiskwhisk until thoroughly mixed. Heat over medium heat. Add egg yolks and condensed milk. Whiskwhiskwhisk some more. Cook custard over medium heat until bubbly and thick. Remove from heat and whisk in butter and vanilla.
Cool custard for a 10-15 minutes. Pour into prepared crust over sliced bananas. Top with remaining sliced bananas and sprinkle with chocolate chips. Cool 4-6 hours. Serve with sweetened whipped cream.