Preheat your fan oven to 160°C (325°F) or to 175°C (350°F) if you're not using a fan oven. Line your cupcake pan with paper cases.
For crunchy base, combine crushed biscuits with melted butter - just like you would for a cheesecake. Add about a tablespoon in each paper case and press it down with something small, I used a small rolling pin.
Sift and combine flour, salt, baking powder and cinnamon to a medium sized bowl and combine with crushed biscuits. Set aside. These are your dry ingredients. In a measuring jug, using a whisk, mix together sour cream and vanilla extract. These are your wet ingredients.
In a bowl of a stand mixer beat together butter and sugar until paler and fluffier, approx. 2 minutes. Add eggs one at a time, mixing well after each addition. Add mashed ripe banana and mix until everything is combined.
Add your dry and wet ingredients, alternating between one another, starting and finishing with dry ones. (3 additions of dry and 2 of wet ingredients)
Divide the batter evenly among the paper cases, filling them ¾ of the way full. Bake for 18-22 minutes or until they're springy on the top. You may have some leftover batter because of the crunchy base.Once they're done, let them cool completely before frosting.
for chocolate ganache sauce
Place finely chopped dark chocolate in a medium sized bowl and pour over it almost boiling heavy cream. Cover it with a plate and leave it for 2-3 minutes. Stir with a silicone spatula to combine.
for vanilla biscuit buttercream
In a small saucepan combine flour, sugar, salt and milk. Cook over medium-high heat, stirring constantly with a whisk until it thickens. After it has boiled, cook for another 2 minutes.
Once it's done, transfer it to a bowl, put a cling film over it touching the surface to prevent "skin" forming and let it cool to room temperature. This is your pudding base. You can do this in a fridge, but before using it let it come to room temperature.
Using a stand mixer fitted with the paddle attachment, beat the butter for 5 minutes until it's pale and fluffy. Then add spoonful by spoonful of previously made pudding. Mix until it's all nicely incorporated and with no lumps. Add the vanilla extract with crushed biscuits and beat for another 30 seconds or until everything is combined.
Assembly:
Using an apple corer, core the middle of each cooled cupcake and fill it with chocolate sauce. Put the buttercream in a piping bag fitted with 2A tip (or whichever you prefer :) ) and frost the cupcakes. Cover with chocolate ganache and decorate with some biscuits. They are best eaten at room temperature. You can keep them in an airtight container at room temperature for 2 days or longer in the fridge. Just let them come to room temperature before eating them. Fridge makes cakes and cupcakes drier so bear that in mind.
Notes
You can use a hand mixer if you don't have a stand mixer.
If you still don't own one, I advise you to buy a kitchen scale - they're cheap and easy to use, and always guarantee the same results in baking! :)
Not all ovens are the same. I suggest to check the cupcakes at 18 minute mark and then adjust the baking time accordingly. Get to know your oven! ;)