In a heavy medium sauce pan, add cornstarch and water, stir to combine. Cook over medium heat till cornstarch is dissolved.
Stir in the condensed milk and whisked egg yolks.
Cook until the mixture is thickened and starts to bubble.
Remove from the heat and add in the butter and vanilla. Stir till combined.
Allow to cool for about 30 minutes.
Fold in the 1/2 cup of toasted coconut.
Slice the bananas and dip them in the lemon juice.
Arrange bananas on the bottom of the pie crust.
Pour custard filling over the top of the bananas. Spread the top evenly.
Cover the pie and chill for 4 hours or overnight, till set.
Spread whipped cream over the top when ready to serve.
Sprinkle toasted coconut over the top, and arrange a few sliced bananas.
Cover and refridgerate leftovers for 2-3 days.