Grease a 20 cm (8 inch) springform tin with butter. You could also make this recipe using a 9-10 inch (22-26 cm) tart base.
For the base, crush the biscuits using a food processor, which makes for a smoother crust. Alternatively place the biscuits in a ziplock bag and crush them with a rolling pin.
Tip the crumbs into a bowl, and pour in the melted butter, that I just melted in the microwave, stirring after every 20 seconds or so or until completely melted- you can also do this over a stovetop.
Mix the ingredients together until they resemble the texture of wet sand.
Pour the base ingredients into your prepared tin and press down firmly using the back of a spoon, or your fingertips. I also like to press the base up the sides, around 2-3 cm (1-1.5 inches).
Set the base aside.
To prepare the filling, bring the heavy cream to a boil on the stovetop or in the microwave (this will take 1-2 min).
Pour the heavy cream over the chocolate chunks, and add the instant espresso powder if using.
Let the mixture sit for around 30 seconds, then stir it using a whisk. It will look grainy at first but will come together.
Add the cubed butter and a pinch of salt, and continue to whisk until the butter has completely melted into the chocolate.
At this stage, you can either pour the filling into the tart shell and chill the tart in the fridge for several hours (2-3 should suffice) or until the filling has set.
Or you can get a bit fancy and pipe it. In this case, pour the chocolate ganache into a container, seal it and place it in the fridge for several hours or until firm and pipeable.
Spoon the mixture into a piping bag fitted with a 1M star nozzle, and pipe rosettes into the base until all the ganache has been used. If you don't have a 1M nozzle, you can also just pipe dollops of the ganache into the shell. And if all goes wrong, just take a knife and spread it out.
If you like, dust the top with some cocoa powder and serve with fresh berries and cream.
Store the tart in the fridge, it will keep for around 1 week.