Heat the oven to 180oC.
Put your oat milk and vinegar in a jug, leaving for a few minutes.
In another bowl, put 1 tbsp vanilla essence, the flour, 150g of your margarine, caster sugar and baking powder. Mix together.
Pour your cake mixture into your milk/vinegar mixture, and beat until smooth (an electric whisk works best!).
Grease 2 x 7in round tins and divide your mixture between them.
Bake in the oven for 30 minutes approx. Leave to cool.
Now, make your filling. For the buttercream, mix your remaining 100g margarine with your icing sugar and 1 tbsp vanilla essence. Beat together until fluffy, not runny.
Spread your jam, then buttercream, onto the top of one of your cakes. Place your other sponge on the top. Dust with a little icing sugar and you're done! Enjoy!