Whisk together the corn starch, salt, egg yolks, and 2 tablespoons of sugar in a small bowl. Set aside.
Combine the heavy cream, whole milk, and half a cup of sugar in a medium, heavy bottomed, saucepan.
Bring just to a boil over medium-high heat, stirring almost continuously. Turn heat to low.
Vigorously whisk about one cup of the hot milk mixture into the egg mixture. Pour the egg mixture into the saucepan and continue whisking for another 2 to 3 minutes until the custard thickens.
Remove from heat and stir in the butter and extract until combined.
Cut the bananas into one-quarter inch slices and toss with the lemon juice. Layer the banana slices over the bottom of the prepared crust.
Spread the custard over the bananas and cover with plastic wrap. The plastic wrap needs to come in contact with the entire top of the custard so it doesn't form a skin. Just press on it gently.
Refrigerate the pie for 3 to 4 hours.
Whip the cream with the vanilla and powdered sugars until stiff peaks form. Spread the whipped cream onto the pie and top with Nilla wafer crumbs and fresh banana slices if desired.