Preheat the oven to 350ºF. Lightly oil a 9-inch cast iron skillet with coconut oil and set aside.

First, make the cake batter. In a small bowl, whisk together the ground flaxseed with 3 tablespoons water and set aside to thicken. (This flax "egg" will help bind the cake together.) In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside for a moment.

Place the coconut oil and 1/3 cup sugar in the bowl of an electric mixer fitted with the paddle attachment, and beat together until light and fluffy. Add in the flax "egg", lemon zest, and vanilla, and mix again. Finally, add the coconut cream, and beat together until the mixture is smooth. Add in the dry ingredients, starting on low speed, and beat well until well-incorporated. The batter will be slightly thick, but still spreadable.

Scrape the batter into the prepared skillet and smooth out with an small offset spatula or the back of the spoon.

Now, it's time to cut and arrange the fruit. Cut each plum in half, removing and discarding the pits. If the plums cling to their pits, carefully cut around the pits instead. Cut each plum half into 1/2-inch thick slices. Arrange the plum slices in the pan however you like: I usually do two concentric circles. Gently press the plum slices down into the batter a bit, to anchor them. Sprinkle the top of the cake

Transfer the pan to the preheated oven, and bake for about 30 minutes, until the cake is golden-brown and starting to pull away from the sides of the skillet. Remove from the oven, allow to cool for at least 15 minutes, then slice and serve warm or at room temperature, with or without your favorite vegan ice cream.