Preheat the oven to 180ºC (fan) / 350ºF / Gas Mark 4. Prepare a 12-hole muffin tray by lining each hole with a paper muffin case.
In a food mixer or large bowl beat together the margarine and sugar until pale and fluffy, then beat in the yoghurt, milk and vanilla extract until smooth. Sift in the flour and bicarbonate of soda and fold in slowly until just combined.
Divide the mixture between the 12 cake cases, then bake for 15-18 minutes until an inserted skewer or knife comes out clean. Leave to cool on a wire rack, and if possible leave in an airtight container overnight to ice the following day.
In a food mixer or large bowl, beat together the margarine and icing sugar, add the vanilla extract and milk and beat again until pale and fluffy. If the buttercream is a little stiff, add a few drops more milk and beat again.
Pipe a generous swirl of buttercream on the top of each cupcake.