Step 1: Place the whole mandarins in a large pot and cover with water. Bring to a boil, then reduce heat and simmer for 1 hour until very soft.
Step 2: Drain the mandarins and let them cool. Once cool, cut them in half and remove any seeds. Place the whole fruit (including skins) into a food processor with the pitted dates and pulse until a thick, relatively smooth paste forms.
Step 3: Preheat your oven to 320°F (160°C). Grease and line a 9-inch (23cm) springform cake tin with parchment paper.
Step 4: In a large bowl, whisk the eggs and sugar together for 3-5 minutes until the mixture is thick, pale, and has doubled in volume.
Step 5: Gently fold the mandarin and date puree into the egg mixture. Once combined, fold in the almond meal, baking powder, and salt until just incorporated.
Step 6: Pour the batter into the prepared tin and smooth the top. Sprinkle half of the chopped pistachios over the surface.
Step 7: Bake for 45-55 minutes, or until a skewer inserted into the center comes out clean. If the top browns too quickly, cover loosely with foil.
Step 8: While the cake is still warm, brush the top with warmed honey and sprinkle the remaining pistachios. Allow the cake to cool completely in the tin before serving.