Genoise Sponge: Preheat oven to 175 degrees Celcius and lightly grease the bottom of a 9in round cake pan.
Using a hand mixer, whisk constantly the eggs, vanilla and sugar over a double boiler with simmering water until the mixture reaches ribbon stage´ about 8-10 minutes. (To determine if you have achieved the 'ribbon stage', the mixture has to be stiffen and thickened as you pull the batter up with the beater and it stays atop the rest of the mixture for about 10 seconds or more, forming a ribbon-like effect. It should be tripled in volume)
Fold 1/4 of the egg mixture into the melted butter to lighten the butter. Fold the mixture back into the rest of the egg mixture. Sift 1/3 of the flour onto the egg mixture, and then gently but quickly fold together. Repeat this steps until the flour has been incorporated. (Take precaution on the folding method as how you fold the mixture together is crucial to determine the success of a light and spongy génoise. You want to evenly incorporate the flour into the egg mixture without deflating the aerated eggs. It´s important that you are quick and gentle at the same time, cutting the spatula down toward the center, lifting the batter from the bottom of the bowl, and turning the bowl with your other hand towards the hand that is folding)
Gently pour the batter into the cake pan and bake about 25-30 minutes or until the cake is golden brown and starts to shrink from the sides of the pan. (Do not open the oven door before the minimum time or the cake could collapse.) To determine the doneness, press the centre of the cake with a finger and ensure the cake will have the springback effect after pressing. Remove cake from oven and place on wire rack to allow the cake to cool completely.
Slice the cake into 3 horizontal layers.
Lemon Syrup: Bring to a boil the sugar, water and lemon zest. Remove from the heat and cool.
Strain of the zest and refrigerate until you are ready to use it. (Syrup can be refrigerated for one month in an airtight container.)
Crème Mousseline (Rich Buttery Pastry Cream): Scald the milk with half the sugar in a heavy bottom saucepan over medium high heat.
While the milk and sugar are heating, whisk together the egg yolks, vanilla extract, the remaining sugar and custard powder / cornstarch until the mixture becomes pale in color.
Once the milk is scalded, bubbling at the edges, slowly pour about 1/4 of it into the egg yolk mixture, whisking until combined. Continue adding the milk in fourths until all has been whisked together. Check the bottom of your saucepan. If there is any scorched milk or film, use another heavy bottom saucepan; otherwise, pour the mixture back into the pan and bring to a boil over medium high heat, stirring vigorously and constantly. Once the mixture boils, continue stirring vigorously over the heat for about 30 seconds more. Remove from the heat continuing to stir until the steam dissipates. Press through a sieve for a smooth cream. Dot the cream with half of the butter in tablespoon-sized chunks. Let the butter melt, and then stir in. Cool over an ice bath for 10 minutes. Cover with plastic wrap. The plastic should come in direct contact with the cream, so a film doesn´t develop. Refrigerate until cold, at least 30 45 minutes.
With a spoon, mix the remaining butter until it´s a soft consistency. Remove the pastry cream from the refrigerator. In a stand mixer, whisk until it's soft. Add 1/4 of the softened butter at a time and combine before adding the next 1/4 of the butter. Place in a pastry bag fitted with a round tip that is 1/2in in diameter.
To Assemble the Fraisier: Place one sliced half of the génoise cake into the bottom of a 9in (23cm) spring form pan. Using a pastry brush, brush about half of the simple syrup onto the top of the cake. (You know you have enough syrup in the cake if you press down on the cake and hear a squish, like that of a full sponge.)
Sort through the strawberries to select 15 of similar size; reserve few strawberries for the top finishing. Trim off the bottoms in order for them to stay flat and are the same height. Reserve the bottom pieces to use in the filling. Cut the 15 strawberries vertically in half. Reserve those strawberries you don't use. Arrange the halved strawberries with their pointed tips on top and the flat side facing out and pressed against the side of the cake ring. Continue until you've lined all sides of the cake ring, which should take about 15 strawberries (slice more if you need to).
When the cake ring is completely lined with strawberries, use an offset spatula to spread in enough crème mousseline to come almost all the way up to the top of the strawberries. Press the crème mousseline against the strawberries to fill any gaps along the sides of the cake ring.
Chop the strawberry bottoms and any leftover strawberries-except the few strawberries reserved for the top-and spread these over the crème mousseline.
Add more crème mousseline and press down against the strawberries to fill in any gaps between the strawberries along the sides of the cake. You should have about 1/2 cup crème mousseline remaining for the top of the cake.