Preheat the oven to 180C/350F/Gas 4.
Grease, flour and line the base of a 23cm/9in spring-form or round loose bottom cake tin.
Place the sugar, eggs and lemon zest in a large bowl set over a pan of simmering water.
Using an electric hand whisk, whisk the mixture over a medium heat until doubled in volume and pale in colour. The mixture is at the right stage when it forms a ribbon trail when the whisk is lifted out of the mixture. Remove from the heat.
Sift in two-thirds of the flour and gently fold into the whisked mixture with a metal spoon or spatula. Add the remaining flour and fold again. Try to keep in as much of the air as possible. Make sure all the flour is incorporated into the mixture.
Gently fold in the melted butter.
Pour the mixture into the cake tin and bake for 25-30 minutes or until the sides of the cake begin to come away from the tin and it is pale golden-brown.
When cooked, allow the sponge to cool a little bit in the tin, then turn out onto a cooling rack. Be careful as this sponge is quite delicate. It should be just under 5cm/2in in height.
To make the crÃ¨me pÃ¢tissiÃ¨re, pour the milk into a wide based pan, split the vanilla pod along its length using a sharp knife, and add it to the milk along with the vanilla seeds. Bring the milk up to the boil, then take it off the heat.
Whisk together the eggs, sugar, kirsch and cornflour in a medium sized bowl until blended.
Remove the vanilla pod from the milk and pour the hot milk through a sieve into the egg mixture. Whisk to combine.
Pour the custard back into a clean saucepan and set over a medium heat.
Stir the custard constantly until the mixture thickens. The mixture will take about four minutes to thicken, but when it does it happens very quickly, so you need to really keep stirring to prevent lumps. Whisk until smooth.
Cook the mixture until the crÃ¨me is very thick, so that it can be piped and it will hold its shape. Stir in the butter until thoroughly melted and combined.
Allow to cool slightly, pour into a shallow dish and chill in the fridge for about an hour until really cold and set firm. This chills it faster as it cools over a larger surface area â€“ alternatively you could fill the piping bags with it at this stage and leave overnight to chill.
Place the ingredients for the lemon syrup in a small saucepan with 70ml/4Â½ tbsp water. Heat gently until the sugar dissolves, then boil rapidly for two minutes. Remove from the heat and set aside to cool.
Meanwhile, roll out a thin disc of marzipan to fit a 23cm/9in circumference circle. It is best if you draw around the 23cm/9in base of another loose bottomed tin for the perfect circle. For best results and a perfectly flat surface, chill it in the fridge until it is needed.
Slice the sponge in half horizontally, creating two slim discs of cake. The cut must be as level as possible as it will be visible in the finished cake.
Place a strip of acetate plastic around the inside of the springform tin. Or line the base and sides with cling film or parchment lined foil.
Place one layer of sponge cake in the bottom of the cake tin. Then liberally brush the sponge with half the syrup. With the back of a spoon, gently squash the edges of the cake down so that they are pushed directly against the sides of the tin, creating the defined edges necessary for the Fraisier cake.
Rinse, hull and halve about 12 strawberries, try and make sure they are all the same height.
Place the cut sides of the strawberries against the plastic on the inside of the tin. The strawberry halves should be sitting snugly beside each other, so it looks like a little crown inside the tin.
Take the chilled crÃ¨me pÃ¢tissiÃ¨re out of the fridge and spoon two thirds of the crÃ¨me into a piping bag, fitted with a 1cm/Â½in nozzle.
Pipe a swirl covering the exposed sponge completely in the bottom of the tin.
Then pipe between each of the strawberries so the gaps are filled right to the top with the crÃ¨me pÃ¢tissiÃ¨re.
Set about 3-5 strawberries to one side for decoration, then hull and quarter the rest of them and place on top of the crÃ¨me, so it raises the inside of the cake by about an inch.
Pipe another swirl of crÃ¨me pÃ¢tissiÃ¨re on top of the cut strawberries to cover the whole surface. Then smooth with a palette knife.
Place the other disc of sponge on top of this, with the cut side uppermost, so it has a completely flat top. Brush with the remaining syrup.
Gently press the top down quite firmly, so that the cake and filling push against the acetate to create the distinctive smooth and defined sides of the Fraisier cake.
Lay the chilled marzipan circle on top of the cake and put the whole thing back in the fridge to set.
Make some pretty decorations of your choice with melted dark chocolate.
When ready to serve, remove the cake from fridge.
Very carefully release the spring tin/loose bottom and remove the cake from the tin and from the acetate or cling film.
Place onto a serving plate and decorate with reserved strawberries, chocolate decoration and a dusting of icing sugar. Serve chilled.