Preheat the oven to 180°C (350°F or gas mark 4). Line the bottom of an 8-inch (20-cm) loose-bottom tart tin.
To make the pastry: Combine the flour, oil, and sugar in a medium-size bowl. If the mixture is crumbly, add some extra water and mix until it becomes a pliable dough. Firmly press the mixture into the tart tin, and smooth the surface. Bake for 15 to 20 minutes, or until the surface is light brown. Allow the pastry to cool in the tart tin.
To make the ganache: Add all of the ingredients in a medium-size saucepan or double-boiler over medium heat. Stir and mix until the majority of the chocolate has melted. Remove the saucepan from the heat and set aside until all of the chocolate has melted. Mix the ganache until all of the ingredients are well combined. Set aside the chocolate in the fridge for at least 2 hours.
Using a stand mixer or electric hand beater, whip the chilled ganache until it is fluffy and has changed colour.
Scoop the ganache into a piping bag with a large star nozzle. Pipe the ganache into the chilled pastry case as desired. Chill the tart in the fridge until ready to serve.
The tart can be kept in an airtight container in the fridge for up to 5 days or in the freezer for up to 1 month.
*If you prefer to make your own chocolate, use around 1 1/2 cups (250g) roughly chopped cacao butter, 1 cup (100g) raw cacao powder or cocoa powder and 2/3 cup (250g) rice malt or maple syrup