Prepare the Cake Tin: Preheat the oven to 180°C (160°C fan). Grease a 23 cm springform or deep loose-bottomed round cake tin, line the base with baking paper, then lightly flour the inside, tapping out any excess.
Whisk the Eggs: Place the eggs, sugar, and lemon zest in a large heatproof bowl set over a pan of simmering water. Using a hand-held electric mixer, whisk for about 5–7 minutes, until the mixture is very thick, pale, and more than doubled in volume.
Check the Ribbon Stage: Lift the beaters from the egg mixture and check that the mixture falling from them leaves a visible ribbon-like trail across the surface before slowly disappearing.
Fold in the Flour: Sift two-thirds of the flour over the egg mixture and gently fold it in using a metal spoon. Add the remaining flour and fold again carefully, taking care to retain as much air as possible while ensuring no dry flour remains.
Incorporate the Butter: Gently fold the melted, cooled butter into the sponge mixture until evenly incorporated without unnecessarily deflating the batter.
Bake the Genoise: Pour the batter into the prepared tin and bake for 25–30 minutes, until pale golden brown and the sides of the sponge begin to pull away from the tin.
Cool the Sponge: Leave the sponge in the tin for about 5 minutes, then carefully turn it out onto a wire rack and allow it to cool completely. Wash and dry the cake tin for assembly.
Infuse the Milk: Place the milk and split vanilla pod in a wide saucepan. Bring the milk just to the boil, remove it from the heat, and leave it to infuse for 10 minutes.
Prepare the Vanilla: Remove the vanilla pod from the milk and scrape out its seeds. Return the seeds to the milk, discarding the pod.
Mix the Custard Base: In a medium bowl, whisk together the eggs, egg yolks, sugar, kirsch, and cornflour until the mixture is smooth.
Combine the Milk Mixture: Gradually pour the warm vanilla-infused milk into the egg mixture, whisking well as you do so. If a skin has formed on the milk, discard it before adding the milk.
Cook the Crème Mousseline Base: Pour the mixture back into the saucepan and cook over medium heat, stirring constantly, until it comes to a boil and thickens. This should take about 4–8 minutes. Keep stirring continuously so the mixture remains smooth and does not catch on the bottom of the pan. Add the Butter: Stir the diced room-temperature butter into the hot thickened mixture until completely combined, then remove the saucepan from the heat.
Chill the Crème Mousseline: Allow the mixture to cool slightly, then transfer it to a shallow dish or bowl. Press cling film or a piece of dampened baking paper directly against the surface to prevent a skin from forming. Refrigerate for at least 1 hour, until cold, firm, and set.
Make the Lemon Syrup: Place the water, sugar, and lemon juice in a small saucepan. Heat gently until the sugar has completely dissolved, then boil for 2 minutes. Remove from the heat and allow the syrup to cool.
Prepare the Marzipan: Lightly dust the work surface with icing sugar and roll the marzipan into a thin 23 cm disc. Keep it chilled until needed.
Slice the Sponge: Once completely cool, cut the genoise horizontally through the middle to create two thin, even sponge discs.
Line the Tin: Place the acetate strip snugly around the inside of the cake tin, positioned between the tin and the sponge. Cling film or parchment-lined foil can be used as an alternative.
Position the First Sponge: Place one sponge disc in the lined tin with the cut side facing upward. Brush the surface evenly with some of the cooled lemon syrup.
Secure the Sponge Edges: Using the back of a spoon, gently press the edges of the sponge outward so they sit directly against the sides of the tin.
Arrange the Strawberries: Select 12 strawberries of approximately equal height and cut each one vertically in half. Arrange them upright around the edge of the sponge, with the cut sides facing the acetate and the pointed ends upward, fitting them closely together. Pipe the First Cream Layer: Transfer about two-thirds of the crème mousseline to the piping bag and pipe a spiral over the sponge to cover it completely. Pipe additional cream into the spaces between the strawberries so there are no gaps.
Prepare the Strawberry Filling: Reserve 3–5 strawberries for decoration. Cut the remaining strawberries into quarters or smaller pieces and distribute them evenly over the crème mousseline.
Add the Second Cream Layer: Pipe another layer of crème mousseline over the strawberries, then smooth the surface level using the back of a spoon or a palette knife.
Add the Top Sponge: Place the second sponge disc on top with the cut side facing upward. Brush it with the remaining lemon syrup.
Compress the Layers: Gently press the top sponge down into the crème mousseline so the assembled cake is firmly held against the acetate around the entire edge.
Add the Marzipan: Place the chilled 23 cm marzipan disc over the top of the cake.
Chill the Cake: Refrigerate the assembled Fraisier Cake for at least 2 hours or overnight, allowing the layers and crème mousseline to firm.
Decorate and Serve: Remove the chilled cake from the tin, top it with the reserved fresh strawberries and any desired decorations, then slice and serve.
