Preheat oven to 210°C and dust a 40 x 30cm piece of baking parchment with icing sugar.
Roll out the puff pastry on the parchment until it is 2cm larger than the paper on all sides.
Place pastry on a tray, dust with icing sugar, prick with a fork, and bake under a second tray for 25-30 minutes.
Trim the warm pastry into three 11 x 24cm rectangles and leave to cool completely.
Cook strawberries and lemon juice for two minutes, then dissolve the jam sugar.
Boil the mixture for 4-5 minutes until it reaches setting point.
Test the jam for wrinkles on a cold plate and transfer to a bowl to cool.
Whisk egg yolks, caster sugar, flour, and cornflour while heating milk with vanilla seeds to a simmer.
Gradually whisk the hot milk into the egg mixture and cook over low heat for 8-10 minutes until thick.
Spoon custard into a bowl, cover with film, cool, and then fold in whipped double cream.
Layer pastry with jam and custard twice, then top with the final pastry rectangle.
Mix 200g icing sugar with lemon juice and water to create a smooth glacé icing.
Combine remaining icing sugar with cocoa and water to create a chocolate paste in a piping bag.
Spread white icing over the top and pipe four thin lines of chocolate icing lengthwise.
Drag a skewer through the icing at 2cm intervals to create a feathered effect.
Chill for several hours or overnight before slicing into six pieces with a sharp knife.