Grease and line the base and sides of a 23cm springform cake tin with greaseproof paper. Preheat the oven to 180°C/160°C fan/gas mark 4. Bring a large pan of water to the boil. Melt the butter and leave to cool.
Place the eggs and sugar in a large heatproof bowl that will fit snugly on top of a pan of water. Remove the pan of water from the hob once it has come to a rolling boil and place on a heatproof surface. Place the bowl over the pan of water, making sure the bottom of the bowl doesn’t touch the water, and whisk using a handheld electric whisk on a low speed for 4 minutes, until pale and tripled in volume. Increase the speed to medium and whisk for another 5 minutes, until the mixture forms a stable ribbon when the whisk is lifted out.
Remove the bowl from the heat and sift the flour over the mixture. Very carefully fold in using a wide metal spoon, making sure not to beat out too much air. Gently fold through the melted butter, making sure there are no flour or butter pockets in the mixture. Carefully pour this mixture into a cake tin and bake for 25 minutes, until pale golden, the sides begin to come away from the edge and a skewer inserted in the middle comes out clean. Leave to cool completely.
Meanwhile, cut the vanilla pods lengthways and scrape out the seeds. Place the milk, vanilla seeds and the halved pods in a large saucepan and heat until it begins to steam. Place the gelatin leaves in a bowl of cold water to soften. Mix together the egg yolks, sugar and flours until well mixed. Gradually pour the warmed milk over, then return the mixture to the pan.
Place over a medium heat and stir constantly for 4-5 minutes. When the mixture becomes very thick and lumpy, beat with a whisk until the lumps are gone and transfer the mixture to a heatproof bowl. Squeeze out the gelatin to get rid of excess moisture and stir it through the still-hot crème pâtissière to make sure it melts. Refrigrate this until needed.
Meanwhile, place the halved strawberries in a small saucepan with the caster sugar over a low-to-medium heat with a lid for 10 minutes, until the strawberries have released their liquid and become very soft. Do not stir. Place the gelatin in a bowl of cold water to soften for 5 minutes. Place a fine sieve over a bowl and pour the strawberry mixture over it, again making sure not to press the strawberries. Stir the gelatin into the warm mixture so that it melts. Set this mixture aside, but not in the fridge.
Place the cooled cake on a chopping board. Using a sharp knife, trim around the edge of the cake so that the sides are completely even. Using a long serrated knife, slice the cake in half horizontally to make two separate layers, then gently trim down the tops so the layers are even.
Place one half of the cake on a cake stand. Place an acetate sheet that is at least 10cm wide around the base of the cake and fasten the edge with tape to ensure that it is completely secure. There should be no gaps around the edge of the cake so the other layers don't leak.
Choose strawberry that are all about 3½ cm in height so the overall result is neat. Cut the stalk of the strawberries off so they have a flat base, then halve them lengthways. Place the strawberry halves on top of the layer of cake, all around the acetate, with the cut half facing the outside, making sure they are all touching so there is a ring of strawberries facing the outside.
Remove the crème pâtissière from the fridge and half fill a piping bag with it. Cut the base of the piping bag so that the hole is roughly ½ cm. Pipe the crème pâtissière between each of the strawberries so that it fills the gaps between them, then continue to fill and pipe the crème pâtissière so that it fills the space in the middle and just covers the tops of the strawberries. Reserve one sixth of the crème pâtissière mixture for later. Carefully smooth the top of the crème pâtissière, making sure it goes right to the edge of the acetate.
Carefully place the second layer of cake on top, then spread over the remaining crème pâtissière so there is a thin layer covering the cake, again making sure that this reaches the very edge of the acetate tape so that the layer of jelly cannot leak out. Pour the jelly over this layer of crème pâtissière in one even layer. Very carefully transfer the cake to the fridge for at least 4 hours, or until the jelly on top has set. Carefully remove the acetate from the outside before serving.