Preheat the oven to 180 degrees celsius / 350 degrees fahrenheit and oil & line two 20 cm / 8 inch round cake tins with baking paper on the bottom and sides.
In a small bowl or measuring jug, pour in the almond milk then add the apple cider vinegar. Give it a brief stir and set it aside for about 10 minutes to activate and become 'buttermilk'. In a large bowl, whisk together the all‐purpose flour, baking powder, baking soda and sea salt. Set aside.
In a large mixing bowl with an electric mixer, or in a stand mixer with the paddle attachment, cream the softened butter until it is a little fluffy and whipped, then add the sugar and vanilla extract and beat until it is combined and fluffy – about 3 minutes. Now begin adding your dry mix & buttermilk to the butter mixture – slowly add 1/3 of the flour mix to the whipped butter whilst mixing on low speed, then add 1/2 of the milk mixture slowly, then another 1/3 of the flour mix, then the remaining milk mix and then finally the last 1/3 of the flour mix. Once you have incorporated the last bit of flour, turn the mixer off, don't over‐mix the batter or it will be dense!
Divide the batter into the two prepared cake pans, then sprinkle the tops evenly with the finely sliced rhubarb. Bake the cakes on the centre rack of the oven for 45–60 minutes, or until a toothpick inserted into the center comes out clean. Remove the pans from the oven and place them on a cooling rack to cool completely before removing from the tins and assembling. In the meantime, prepare the compote and whipped coconut cream frosting.