Preheat the oven to 320°F / 160°C and line a 6 inch 15 cm diameter cake pan with parchment paper (bake in 3 batches).
In a container, mix together the flour, baking powder and salt.
In another container, place the buttermilk and vanilla together and set aside.
Place the soft butter, oil and sugar in a large bowl and beat with a whisk for 2-3 minutes until fluffy.
Then add the egg whites one by one while leaving the mixer running and finish by adding alternately the mixture of dry ingredients and the liquids.
Divide the mixture into 3 parts and bake for about 40-45 minutes (baking in 3 batches).
Let your cakes cool for a few minutes before unmolding them and letting them cool to room temperature on a cake rack.
Cover your cakes with cling film and place them in the fridge for at least 2 hours.
In a saucepan, bring the milk to a boil with the split and scraped vanilla bean, then let it infuse for 10 minutes.
In a bowl, beat the yolks with the sugar, then add the cornstarch.
Remove the vanilla bean from the milk and pour it into the bowl containing the yolks, sugar and cornstarch.
Mix well and then return to the saucepan over medium heat, whisking continuously until the mixture thickens, at the first boil continue to mix for another minute.
Pour the mixture into a dish, cover with plastic wrap and leave to cool completely in the refrigerator.
Place the butter in the bowl of your mixer and whip for 5 minutes at high speed, then add the custard and continue to whip for another 5 minutes, keep your mousseline cream in the fridge until assembly.
Swiss Meringue Buttercream
Place the egg whites with the sugar in the bowl of your stand mixer and place in a saucepan with a base of water.
Heat in a double boiler over medium heat while stirring with a hand whisk, bringing the mixture to 55°C.
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 is completely cool.
Turn down the speed and add the softened butter cut into pieces, vanilla and salt.
Process on high speed for about 10 minutes, the mixture will be liquid at first, then grainy before firming up.
Set aside at room temperature until ready to assemble.
In a double boiler, place the chopped white chocolate with the heavy cream and over low heat melt this mixture. Reserve at room temperature until assembly.
Fill 2 pastry bags with the mousseline cream and the Swiss meringue butter cream.
Place a base on your turntable and place a first layer of sponge cake and place a ring of buttercream on the edges.
Fill the inside with mousseline cream and place slices of strawberries and cover again with mousseline cream.
Place a second layer of cake and repeat this last step, finishing with the last layer of cake.
Cover completely with a thin layer of buttercream and smooth quickly to crumbs coat and place in the fridge for 10 -15 minutes
Apply a new layer of buttercream and smooth with an icing smoother.
Add the white chocolate dripping and spill it over the sides of the cake.
Decorate the top of the cake with the remaining buttercream and strawberries.
Keep in a fridge and take it out 30 minutes before serving it.