Preheat the oven to 180C / 350F / Gas Mark 4. Grease and line two 20cm or 22cm cake tins.
In a food mixer, beat the margarine and sugar together for 3-4 minutes, (yes, really!), until pale and fluffy.
Add the yoghurt, soya milk, and the vanilla extract, then add them to the mixer and beat. (If the mixture looks 'curdled' just add a few tablespoons of the flour to the mix and beat again).
Add the flour and baking powder, (the remainder if you have added some already), and slowly fold it into the mixture, until fully combined.
Divide the mixture between the two tins, and bake for 20-22 minutes until an inserted skewer or knife comes out clean. Then remove from the oven and turn out onto a cooling rack.
To make the buttercream, just beat the margarine, icing sugar and lemon zest in a food mixer for 3-4 minutes until very light and fluffy.
When the cakes are completely cool, (a few hours at least), smother the top of one with plenty of jam, then smother the buttercream over the bottom of the other cake. Carefully sandwich them together, then sprinkle the top of the cake liberally with icing sugar. NB: Slightly bready texture. Will come away from sides of tin when done cooking.
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