To begin you will need to prepare the pistachios by soaking them. Add them to a large bowl, cover with water and refrigerate overnight (or a minimum of 6 hours).
Rinse the pistachios with clean water and remove as much of the skins as possible. They should coming off easily using your fingers as the water will have loosened them
For the crust, add the buckwheat, oat flakes, salt and cacao powder to your food processor and process to a light crumb consistency.
Add the dates, coconut oil and maple syrup and continue to process until the ingredients begin to stick together and form a dough like consistency. If the dough isn't holding together when you press it between your fingers, add a teaspoon or two of water and blend until it sticks.
Line the base of a 9" springform cake tin with parchment paper. Press the dough into the base and push it up the sides of the tin to form a crust. You can use a spatula or the bottom of a flat glass to smooth is out.
Trim around the rim of the crust to make it as even as possible.
Set the base aside in the freezer.
Add all of the filling ingredients to your high speed blender and blitz until completely smooth and creamy without any lumps.
Pour the mixture onto the crust and tap the cake tin off of your work surface to knock out any air bubbles.
Place the tart into the freezer to set for about 2-3 hours.
Once set, carefully remove the tart from the springform tin whilst still frozen.
Decorate the tart by adding chopped pistachios, chopped dark chocolate and rose petals around the outer surface of the cake. Allow to defrost for 30-40 minutes before serving.
This tart can be stored in the fridge in an airtight container for up to 5 days. Alternatively, freeze in sections and defrost as needed.