Add thickened cream to a large metal mixing bowl. Whip on high using a hand mixer until cream thickens to stiff peaks that can hold their shape. Cover with plastic wrap and set aside in the fridge.
Add the custard powder and sugar into a large microwave safe bowl and mix with a whisk to combine. Slowly stir in milk while whisking.
Microwave for 1 min at a time until it thickens. If you feel it's too thick add about 1/4 cup of milk and mix it in. Run it through a sift and add vanilla. Mix in well, cover with plastic wrap directly on the custard and chill for an hour.
Preheat your oven to 180C (356F) or 160C (320F) for a fan forced oven. Line a cupcake tin with cupcake liners.
In the bowl of a stand mixer fitted with the paddle attachment, add the flour, baking powder, caster sugar and salt. Turn mixer on low speed and allow it to mix for a couple minutes to help everything combine well (alternatively you may do this by sifting the ingredients together). Add the softened butter and let it mix until it resembles a fine sand like texture.
Next, add milk, eggs, yogurt, mashed banana, oil and vanilla extract in a large jug and whisk well.
Add wet ingredients to dry ingredients in a slow and steady stream until no dry ingredients are visible. Scrape down the bowl and mix for another 20 seconds.
Fill each cupcake liner 3/4 of the way (using an ice cream scoop to transfer the batter into the cupcake liners makes this a quick and easy process otherwise you can use two tablespoons).
Sprinkle with chopped graham crackers. Try to leave the center free of crackers so you can core the cupcakes. Bake for 20-25 min or until a toothpick inserted comes out clean.
Allow them to cool completely on a wire cooling rack before frosting.
Once your cupcakes have baked and cooled core the center and fill with custard. Fit the end of your piping bag with a round tip and frost with whipped cream in a swirl. You want to work quickly so the cream doesn't melt from the warmth of your hands.
Sprinkle with chocolate shavings. Store in the fridge.