Prepare the pastry cream: In a medium saucepan combine the milk, 1 cup heavy cream, 1/4 cup granulated sugar, and a pinch of salt; bring to a light boil over medium heat, whisking to dissolve the sugar, then remove from heat and let rest a few minutes.
In a mixing bowl whisk together 1/2 cup granulated sugar and the cornstarch until blended; add the whole egg and the 2 egg yolks and beat with an electric mixer on low until pale and slightly fluffy, about 1 minute.
Temper the eggs: while whisking, add about 2/3 cup of the hot milk mixture to the egg mixture to warm it, then pour the warmed egg mixture back into the saucepan with the remaining milk mixture while whisking continuously.
Cook the custard over medium heat, stirring constantly, until it thickens to a pudding consistency, about 4–8 minutes; whisk vigorously near the end to prevent lumps.
Remove from heat, stir in the vanilla, then strain the pastry cream through a fine mesh strainer into a bowl to remove any lumps.
Press plastic wrap directly onto the surface of the pastry cream to prevent a skin, then chill in the refrigerator until fully cold, about 4 hours.
Make the crust: In a bowl combine the graham cracker crumbs and 2 Tbsp granulated sugar, then stir in the melted butter until evenly moistened.
Assemble the cups: Divide the graham cracker mixture among 12 dessert cups (about 2 Tbsp each) and gently press into an even layer.
Add banana slices: Place 3 banana slices on top of the crust in each cup.
Pipe the chilled pastry cream over the first banana layer using a piping bag fitted with a large round tip, smoothing as needed.
Add 3 more banana slices to each cup, then spoon a scant 1 Tbsp caramel sauce over the bananas (warm slightly if too thick, but do not heat).
Make the whipped cream: In a large bowl whip 1 3/4 cups heavy cream until soft peaks form, add 3 1/2 Tbsp sugar and continue whipping to stiff peaks; transfer to a pastry bag fitted with a star tip.
Pipe whipped cream over the caramel layer in each cup and finish with grated bittersweet chocolate if desired.
Chill assembled cups in the refrigerator until ready to serve and garnish with a banana slice just before serving.