In a food processor, combine roasted buckwheat, oat flakes, and Himalayan pink salt, blending until a fine flour forms. Add solid coconut oil, maple syrup, and cashew butter, pulsing until the dough holds together. Line the tart tin with parchment paper and press the dough evenly into the base and sides. Freeze the crust for 30 minutes.

Slice the passion fruits in half and sieve out the flesh to separate the seeds; set the seeds aside. In a blender, combine the passion fruit flesh, mango chunks, chilled coconut milk, vanilla extract, and additional maple syrup. Blend on high until smooth.

In a small glass, mix agar agar powder with mandarin juice, stirring well to dissolve. Pour the blended mango and passion fruit mixture into a saucepan. Heat over medium heat until it reaches a gentle boil, stirring frequently. Once boiling, add the mandarin and agar agar mix, stirring for 2-3 minutes until thickens slightly.

Pour the thick filling over the frozen crust, using a spatula to spread evenly. Gently tap the tart on the counter to release air bubbles. Cover with plastic wrap and refrigerate for at least 1 hour to set.

Remove the tart from the refrigerator. Garnish with additional scoops of passion fruit pulp. Slice into wedges and serve chilled.