To make the base add the buckwheat, oat flakes and salt to your food processor and blitz until finely ground.
Add the remaining ingredients and continue to blitz until the mixture forms a dough.
Line the base of your tart tin with parchment paper.
Press the dough into the base and sides of the tin, flattening and smoothing it out using the bottom of a flat glass.
Place the dough into the freezer whilst you prepare the filling.
For the filling, separate the seeds from the juice/flesh of the passionfruit by passing them through a sieve. Discard the seeds.
Add the passionfruit flesh/juice, mango, coconut milk, vanilla and maple syrup to your blender and blend on high speed until smooth and creamy.
In a separate glass, mix the agar agar with the tangerine or mandarin juice and stir well, set aside.
Add the passionfruit/mango mixture to a saucepan and bring to the boil.
Once boiling, whisk in the agar agar mixture and keep stirring for a minute or until the mixture begins to thicken.
Remove from the heat and pour on top of the cake base.
Tap the cake tin on your work counter to knock out any air bubbles.
Allow to cool slightly before placing in the fridge to set for 1 hour minimum
Garnish the tart with more passionfruit and serve.