Heat the oven to 350°F (175°C). Lightly grease a 9-inch tart pan with a removable bottom. Mix the almond flour, cacao powder, 3 tablespoons arrowroot starch and 1/4 teaspoon salt. Stir in the melted coconut oil, 3 tablespoons maple syrup and 1 teaspoon vanilla until evenly moistened.

Press the mixture firmly and evenly over the base and up the sides of the pan. Bake for 12–15 minutes, until the surface looks set and the edges smell lightly toasted; the dark color makes browning hard to judge. Cool completely on a rack, allowing about 30 minutes.

Put the chopped chocolate in a heatproof bowl. Heat the measured 3/4 cup full-fat coconut milk just until steaming, without a rolling boil. Pour over the chocolate and leave for 2–3 minutes, then stir gently from the center until smooth. If a few pieces remain, warm the bowl briefly over barely simmering water, keeping water out of the chocolate.

Stir 2 tablespoons maple syrup, 1 teaspoon vanilla and a pinch of salt into the smooth ganache. Pour into the fully cooled crust and level gently. Refrigerate for at least 4 hours, or overnight, until firm enough to slice. Cover once cold; keep the tart refrigerated until serving.

While the tart chills, combine the blackberries, pomegranate juice, 2 tablespoons maple syrup and lemon juice in a small saucepan. Simmer gently for 8–10 minutes, stirring and lightly crushing the berries, until juicy and slightly thickened.

If a thicker sauce is wanted, stir the optional 1/2 teaspoon arrowroot into 1 teaspoon cold water. Stir the slurry into the simmering sauce and heat only briefly, about 30–60 seconds, until thickened. Remove from the heat. Leave the sauce textured, or press through a sieve if desired; cool, then refrigerate.

Remove the chilled tart from its pan carefully. Slice with a warm, dry knife, wiping it between cuts. Serve eight slender slices with the cooled sauce alongside; do not pour warm sauce over the whole chilled tart.