To make the cake, preheat the oven to 180C/350F/gas mark 4. Grease a 10cm (4in) tin can or deep cake tin and line the base and sides with baking parchment.
Sift together the flour, cornflour and baking powder, stir in the salt and sugar into a bowl. Add the butter and beat on a low speed with an electric mixer until all the butter has been combined, the mixture should have a fine crumbly, sandy texture.
In a jug, whisk together half of the milk, the vanilla extract and the lemon juice. In a separate jug, whisk together the egg white and other half of the milk.
Increase the mixer speed to medium-low and gradually add the vanilla milk mixture, beat for a minute until well combined. Gradually add the egg white mixture a little at a time; beat for a couple of minutes, stopping to scrape the sides and bottom of the bowl occasionally. The mixture may look a little curdled, don't worry.
To make the whipped white chocolate ganache, place the chopped chocolate in a heatproof bowl and heat the cream until just boiling. Pour the cream over the chocolate, leave for a minute then stir until smooth. Set aside in the fridge until it is cold but not set.
Whisk the cooled ganache with an electric mixer until thick and spreadable but not completely stiff, be careful not to overwhisk it.
Carefully trim the top of the cake to level it and then slice it in half (I used a cake wire). Place the bottom layer on a serving plate/small cake stand and spread over some of the whipped ganache. Press some berries into the ganache then place the other cake half on top.
Spread the rest of the ganache over the top and sides of the cake then place it in the fridge while you make the drip.
As before, place the chopped chocolate in a heatproof bowl and heat the cream until just boiling. Pour it over the chocolate and stir until smooth. Stir in a drop of gel food colouring until it reaches the desired shade the set aside until it has cooled and thickened slightly but is still pourable.
Use a spoon to carefully pour the ganache over the cake, starting at the edges then filling in the middle. Return the cake to the fridge for 10 minutes to firm up a little then top with berries and serve. Best eaten the day it is assembled. Keep refrigerated.