Crush the digestive biscuits in a food processor until fine crumbs form.
Pour the melted butter into the crumbs and mix until well combined.
Press the biscuit mixture into the base and up the sides of a tart tin, using the back of a spoon to press firmly.
Place the tin in the fridge to chill while you prepare the filling.
In a saucepan, heat the double cream and caster sugar over medium heat until it starts to simmer.
Remove from heat and add the chopped dark chocolate and vanilla extract to the cream mixture.
Stir until the chocolate is completely melted and the mixture is smooth.
Pour the chocolate ganache into the chilled biscuit base.
Smooth the top with a spatula.
Chill the tart in the fridge for at least 2 hours, or until the ganache is set.
Once set, remove from the fridge and serve chilled.
