Position a rack in the center of the oven and preheat to 350ºF.
In the bowl of a stand mixer fitted with the paddle attachment, combine the sweet rice and almond flours with the cocoa powder, tapioca starch, sugar and salt. Scatter the butter pieces over the top and drizzle with the vanilla extract. Turn the mixer to medium-low and run until the dough comes together in clumps and the butter is worked through, 3–5 minutes. (It will seem as though the dough won’t come together, but don’t worry – it will!)
Dump about half of the crumbs into a 5x13-inch rectangular loose-bottom tart pan (or 9-inch round tart pan) and press evenly into the sides of the pan. Add the remaining crumbs and press evenly into the bottom – it usually takes me about 10 minutes to make it look pretty.
Place the pan on a rimmed baking sheet to catch any drips and bake until slightly puffed and firm to the touch, 20-25 minutes. Remove the crust from the oven and, while it’s still hot, press the sides and bottom with the back of a spoon. This will help it hold together when cool. While the crust is still hot, scatter the chocolate in the bottom and let sit a few moments to melt. Use the back of a spoon or an offset spatula to spread the chocolate over the bottom and sides of the crust. Let cool completely.
Make the filling:
In the bowl of a blender (preferably high-speed) or food processor, combine the soaked and drained cashews, cocoa powder, ground chia seed, maple syrup, water, salt, and vanilla. Blend until very smooth, starting on low and gradually increasing to high, about 2 or 3 minutes in a high-speed blender and longer if needed. Add the coconut oil and blend briefly until smooth. Pour the filling into the crust; you’ll have about a cup left over to chill and eat as pudding. Chill the tart until firm, at least 4 hours and up to a day or two.
Make the topping:
When ready to serve, remove the sides from the tart pan and place the tart on a serving board or platter. Without tilting or shaking the can, remove the coconut cream from the refrigerator and carefully open. Scoop the hardened cream into the bowl of a stand mixer fitted with the whip attachment (or into a large bowl with an electric beater), leaving behind and discarding the clear liquid. Whip on high speed until fluffy and mounded like softly whipped cream, then beat in the maple syrup and vanilla to incorporate.
Spread the coconut cream over the chilled tart and sprinkle with the chocolate shavings and pomegranate, if using. Optionally chill again to firm the cream, half an hour or up to 1 day. To serve, use a large, sharp chef’s knife dipped in hot water and wiped clean between cuts to cut the tart into slices or wedges and serve. The tart will keep, refrigerated airtight, for up to several days.
For the best flavor, I usually make the crust with dutch-process cocoa powder and the filling with raw cacao powder or natural cocoa – I usually have all three lying around (I’m weird). But the recipe will absolutely work using just one type of cocoa if you prefer, so take your pick.The filling is mildly sweet, so feel free to add another tablespoon or two of maple syrup to your taste.The chia seed in the filling helps it emulsify and thicken; if you don't have access to chia seed, flaxseed will *probably* work, it will just have a more pronounced flavor. I buy whole chia seeds and grind them finely in a coffee grinder, then measure out 2 tablespoons of ground seeds.Pomegranate arils on top add a fresh pop of color and flavor, but if they aren’t around, the tart is lovely without them, too. Or top this tart with fresh berries in the spring and summer.Nutritional values are based on one of ten servings.