Mix the oat milk and vinegar, leave to stand for approx. 5 mins. In a bowl, mix the flour with the sugar, baking powder, sodium bicarbonate and salt. Add the coconut oil, vanilla paste and oat milk & vinegar mixture, mix to form a smooth batter. Transfer approx. two-thirds of the batter to the prepared tin.
Mix the oat milk with the cocoa powder, mix into the remaining batter. Spoon the dark batter on top of the light batter. Top with the cherries, press down gently.
Bake for approx. 25 mins. in the centre of an oven preheated to 180°C.
Remove from the oven, leave in the tin and cool on a cooling rack.
Heat the oat milk in a pan, add the pudding powder, stir until smooth, bring to the boil while still stirring. Reduce the heat, simmer the custard for approx. 1 min., stirring constantly. Leave the custard to cool. Using the whisk on a hand mixer, beat the margarine until light and creamy, stir in the custard. The custard and margarine should be a similar temperature, otherwise the mixture will curdle. Spread the mixture over the cake, cover and chill for approx. 2 hrs.
Place the chocolate and coconut oil in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate while stirring, continue to stir until smooth. Spread the chocolate over the custard, leave to stand for several minutes. Create a wave pattern using a fork. Cut the cake into approx. 12 pieces.