Preheat the oven to 180°C/160° fan/350°F/gas mark 4. Grease a 23 x 33 cm / 9 x 13 inch rectangular cake tin and line it with baking parchment.
Sift together the plain flour, cornflour, baking powder, bicarbonate of soda and salt. Make sure they are very well combined.
In a separate large bowl, whisk together the milk, yogurt, melted butter, oil, sugar, vanilla and vinegar until smooth.
Pour the dry ingredients into the wet and whisk until no dry lumps remain. Be careful not to over-beat the batter.
Pour the batter into the prepared tin and spread it level. Bake for about 25 minutes until the cake is firm to the touch and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for 20 minutes then carefully turn it out onto a wire rack.
Spread a generous layer of jam over the top while the cake is still warm then sprinkle with desiccated coconut.
Allow to cool then serve with custard. Store any leftovers in an airtight container at room temperature for up to 5 days.