Preheat the oven to 180C/350F. Line a 20cm / 8 inch round cake pan with baking paper on the bottom & sides, leaving some overhang to use as a handle after baking to lift the cake out of the tin.
In a small bowl, whisk together the ground flax & water, set aside to thicken to make a flax egg. In a separate small bowl, whisk together the coconut milk & the apple cider vinegar. Set aside to thicken/activate & turn into 'buttermilk'.
In a large bowl, whisk together the dry ingredients. Set aside.
Place the chocolate, coconut oil & espresso powder in a large heat‐proof mixing bowl & place over a saucepan filled with a few cm of simmering water. Allow to melt, stirring often so that the chocolate does not burn. Once melted & combined, remove from the heat (it only needs to just melt, don't leave it any longer or it will burn).
Add the sugar straight away while the mix is still warm & whisk well to begin dissolving the sugar. Whisk in the vanilla extract followed by the flax egg & finally the buttermilk. Whisk together until well combined.
Pour the wet mixture over the dry mixture & fold until just combined. Scoop the batter into the prepared pan & spread with the back of a spoon until even. Place the plum / pluot halves on top of the cake, cut side up. Space them out evenly, they shouldn't be too close together. Sprinkle with a little sugar then place in the oven on the middle rack to bake.
Bake for about 60–80 minutes until crackly on top & a skewer inserted into the middle comes out with some crumbs attached. If it's still gooey just leave it for 5–10 more minutes & test again. If the top starts to burn at any time during the baking period, just lower the oven temp to 160 celsius / 320 fahrenheit and continue baking. Once cooked, remove from the oven & set the cake tin on a wire rack to cool for 20 minutes. Then remove the cake from the tin and leave to cool on the wire rack till completely cool or just slightly warm. Serve in thick slices with the espresso whipped cream.