Preheat the oven to 170°C (fan) / 325°F / gas mark 3. Line a 12-hole cake tin with paper cases.
In a food mixer or large bowl, beat together the margarine and caster sugar until pale and fluffy, (this might take 2-3 minutes - keep going!).
In a small bowl, mix the vegan yoghurt, plant milk and vanilla extract.
Add these to the mixing bowl along with the flour and bicarbonate of soda and mix slowly until fully combined.
Divide the mixture between the 12 cake cases then bash the cake tin firmly down on the work surface to bring any small bubbles to the surface. Bake for 13-15 minutes until an inserted knife or skewer comes out clean. Leave to cool on a wire rack and if possible, leave in an airtight container overnight to decorate the following day.
Sift the icing sugar into a small bowl and add the water a little at a time, mixing well until it has reached a thick, spoonable consistency. You may not need all the water, so stop as soon as it is ready. Spoon a little onto the centre of each cake and push it around with the spoon to spread a little, then immediately cover with sprinkles.