Preheat the oven to 180C/350F/gas mark 4. Grease three 15cm/6 inch round cake tins and line the bases with baking parchment (or use two 20cm/8 inch tins instead).
In a large jug, whisk together the plant milk, vinegar, caster sugar, vanilla extract and melted vegan spread.
In a large bowl, sift together the plain flour, cornflour, baking powder and bicarbonate of soda.
Gradually whisk the wet ingredients into the dry until smooth but be careful not to overmix.
Divide the batter between the tins (I use a scale to make sure they are even). Bake for 25-35 minutes until golden and a skewer inserted into the centre comes out clean.
Leave to cool in the tins for 10 minutes then carefully turn the cakes out onto a wire rack and leave to cool completely.
Place the vegan spread and vegetable shortening in a large bowl and whisk with an electric mixer until very smooth.
Add the sifted icing sugar and the vanilla extract. Mix on a low speed until combined then turn the mixer up to a high speed and whisk for a couple of minutes until light and fluffy.
Trim the tops of the cakes if needed. Place one cake layer on a cake stand and spread over some of the buttercream. Spread over a thin layer of jam then scatter over a handful of berries.
Place another cake layer on top and repeat. Place the final cake layer on top and spread with buttercream but not jam. Decorate with berries and serve. Best eaten on the day it is assembled.