Cake : 

1. Preheat the oven to 320°F/160°C and line the bottom of three 6 inch/ 15 cm diameter round cake pans with parchment paper (if you only have one pan, see notes). 
2. In a bowl, sift together the cake flour, baking powder and salt and set aside. 
3. In the bowl of your stand mixer or electric mixer, place the egg whites with the sugar and vanilla and beat at high speed for about 10 minutes until the mixture doubles in volume.
4. Meanwhile, whip the cold heavy cream on medium speed for 4 minutes until you have a whipped cream, then set aside.
5. Lower the speed of the mixer and add the dry ingredients and mix just until the mixture is smooth.
6. Then add the whipped cream and mix this time with a spatula to incorporate it gently and until you have a smooth and homogeneous mixture.
7. Divide the mixture into 3 and pour it into your prepared pans. 
8. Bake in the oven at half height for about 40 - 45 minutes.
9. After 30 minutes of baking, check the cooking with a toothpick by plunging it into the center of the cake, it should come out clean. If you still have traces of liquid batter then extend the baking time by 5 to 10 minutes and check again.
10. Let the cakes cool a bit in their pans before unmolding them and letting them cool completely on a cake rack upside down.
11. Then cover with plastic wrap and place in the fridge. 

Diplomat cream

1. In a saucepan, bring the milk to a boil with the vanilla bean split and scraped, then let it infuse for 10 minutes.
2. In a bowl, beat the yolks with the sugar, then add the cornstarch.
3. Remove the vanilla bean from the milk and pour it into the egg yolk mixture.
4. Mix well and then return the mixture to the saucepan over medium heat, whisk continuously until the mixture thickens, at the first boil continue to mix for another minute.
5. Then pour the mixture into a baking dish, cover with plastic wrap and leave to cool completely in the refrigerator.
6. When the pastry cream has cooled down, whip the cream with the mascarpone in an electric mixer for 3-4 minutes until you have a whipped cream.
7. Mix the pastry cream with the mixer for 1 minute to loosen it then add a tablespoon of whipped cream and beat again.
8. Add the remaining whipped cream to the pastry cream and mix gently with a spatula to incorporate it.
9. Set aside in a cool place until ready to assemble.

Swiss Meringue Buttercream

1. Place the egg whites and Sugar in the bowl of your stand mixer and place in a saucepan with a bottom of water.
2. Heat in a double boiler over medium heat while stirring with a hand whisk, bringing the mixture to 131°F / 55°C.
3. Then place the bowl on the base of your stand mixer with the whisk attachment and beat at high speed for at least 10 - 15 minutes until the meringue is firm and the mixture has cooled completely.
4. Turn down the speed of the mixer and add the softened butter cut into pieces, vanilla extract and salt.
5. Process on high speed for about 10 minutes, the mixture will be liquid at first, then grainy before firming up.
6. Finish mixing with the flat spatula at slow speed for 5 minutes to loosen the buttercream and reserve at room temperature until assembly.

Assembly of the layer cake

1. Fill 2 pastry bags with the diplomat cream and the Swiss meringue buttercream.
2. Place a card board on your turntable and place a first layer of cake and place a buttercream ring on the edges.
3. Fill the inside with diplomat cream and place strawberries cut in 2 and cover again with diplomat cream (if necessary add more buttercream around to enclose the filling in the center)
4. Place a second layer of cake and repeat this last step ending with the last layer of cake.
5. Cover completely with a thin layer of buttercream and quickly smooth with the angled spatula to enclose the crumbs then place in the fridge for 10 - 15 minutes.
6. Apply a new layer of buttercream over the entire cake and smooth it with an icing smoother.
7. Place in the fridge again for 30 minutes before decorating the top of the cake with an icing tip and strawberries.
8. Remove the strawberry layer cake from the fridge about 30 minutes before serving.