Preheat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.
In a medium bowl whisk together the flour, baking powder, salt, and crushed graham crackers for the cupcake base.
In a large bowl beat the softened butter and granulated sugar with a mixer until light and fluffy, about 2 to 3 minutes.
Add the eggs one at a time to the butter mixture, beating well after each addition, then mix in the vanilla extract.
Add the dry ingredients to the wet mixture in two additions alternating with the milk, beginning and ending with the dry ingredients, mixing just until combined.
Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
Bake the cupcakes for 16 to 18 minutes or until a toothpick inserted in the center comes out clean, then transfer the pan to a cooling rack.
Let the cupcakes cool in the pan for 5 minutes, then remove them from the pan and cool completely on the rack.
While the cupcakes bake, prepare the pastry cream by whisking together the egg yolks, sugar, and cornstarch in a medium bowl until smooth.
Heat the milk in a small saucepan over medium heat until steaming but not boiling.
Slowly pour about half of the hot milk into the egg mixture while whisking constantly to temper the eggs.
Pour the tempered egg mixture back into the saucepan with the remaining milk and cook over medium heat, whisking constantly, until thick and bubbling, about 3 to 5 minutes.
Remove the saucepan from the heat and whisk in the butter and vanilla extract until smooth.
If desired, strain the pastry cream through a fine-mesh strainer into a clean bowl to remove any lumps.
Press plastic wrap directly onto the surface of the pastry cream and refrigerate until completely chilled, at least 30 minutes.
Once the cupcakes are cool, use a small knife or the end of a piping tip to cut a small cone out of the center of each cupcake, reserving the tops.
Place a slice or two of banana into each hollowed cupcake center.
Transfer the chilled pastry cream to a piping bag or zip-top bag and pipe it into the banana-filled centers of each cupcake, then replace the small cake tops if desired.
In a large mixing bowl whip the cold heavy cream, powdered sugar, and vanilla extract with a mixer until medium-stiff peaks form.
Pipe or spoon the whipped cream generously on top of each filled cupcake.
Sprinkle the tops with crushed graham crackers and garnish with additional banana slices if serving immediately.
Refrigerate the cupcakes for at least 20 to 30 minutes to set the filling before serving.