Preheat the oven to 320 degrees Fahrenheit (160 degrees Celsius).
Line a springform with baking paper (8 inches / 20 cm diameter).
Mix the flour and baking powder in a large bowl.
Add ½ cups (125 grams) of sugar and the salt to the bowl.
In a second bowl mix the oil, egg yolks, water, vanilla, and lemon zest.
Pour the contents of the second bowl into the first and mix well with a ladle until you get a smooth mixture.
Put the egg whites in another bowl and whisk until stiff with the help of a mixer.
Add the tartaric acid and continue mixing until peaks are formed.
Add the remaining sugar, little by little until the egg whites are firm and shiny.
Spoon about ⅓ of the egg white mixture through the flour mixture with a plastic spatula. Fold it carefully until it is properly incorporated. Then you fold the rest of the egg white mixture through it.
Pour the dough into the springform and bake the cake in the oven for 55 minutes.
Check that the cake is good with a satay stick.
After baking, pour the cake on a rack and let it cool down. Turn the cake over and remove the baking paper. The cake can be baked four days in advance and stored in the refrigerator.
Pour the milk into the pan and add the vanilla and salt.
Warm on medium heat and bring it to the boil (when you see steam, but no bubbles yet), while stirring occasionally.
In the meantime, mix the cornstarch and the sugar in a bowl.
Add the eggs and beat with the whisk until you have a smooth mixture.
When the milk almost boils, pour the hot milk with the cornstarch drip at a time while continuing to beat. Pour the mixture back into the pan (still whisking) and warm until the custard is thickened
Remove from the heat and strain the custard mixture.
Divide the butter into four and add one lump at a time while whisking.
Cover with a plastic foil that touches the top of the custard, so no sheet will form.
Allow the mixture to cool to room temperature.
Sprinkle gelatin on the water and let it soften for a few minutes. Warm the mixture for a minute in the microwave.
Meanwhile, hang a bowl above a pan with a thin layer of boiling water on low heat (au-bain-marie).
Put 60 ml custard in the bowl and heat to approximately 120 degrees Fahrenheit (50 degrees Celsius). Add the gelatin and stir well.
Remove the bowl from the heat and add the rest of the custard. Mix well.
In a second bowl mix the mascarpone with the whipped cream until stiff peaks are formed.
Fold the cream mixture through the custard with a spatula.
Mix the water with the sugar in a small saucepan.
Bring to the boil, and let the sugar dissolve.
Turn the heat of and let it cool down slightly
Line the sides of a springform of 8 inches (20 cm) with plastic foil (not the bottom).
Slice the biscuit, horizontal, in half.
Put the first layer of biscuit in the springform.
Sprinkle 3 tablespoons of the syrup over the cake.
Remove the crowns from the strawberries. Halve as many strawberries as needed to rest against the inside of the springform.
Place the strawberries against the sides of the baking tin on the first layer of cake.
Pipe some buttercream between the strawberries and a thin layer on the cake.
Cut the remaining strawberries into quarters and spread them on top of the buttercream.
Hold two tablespoons buttercream apart and pipe the rest on top of the strawberries.
Place the second biscuit layer on top.
Sprinkle the remaining syrup on the cake.
Smear the buttercream on top of the second biscuit and cover the cake with a plastic foil.
Let stiffen in the refrigerator for at least 4 hours.
Before serving, remove the spring form and gently peel the plastic away.
Sprinkle with icing sugar and serve immediately.
In the recipe, you use cream of tartar to make your cake airy. Cream of Tartar is prepared from natural raw materials, in contrast to baking powder which is made out of chemical substances. The reaction it gives in your batter is the same. Therefore you can replace the cream of tartar with baking powder.
The beaten egg whites are added in two portions and not all at the same time. You do this to let the butter and egg whites mingle better, so the cake will get airier. With a skewer, you can check the doneness of the cake. Prick with a skewer into the center (the thickest part) of the cake. When the skewer comes out dry then the cake is done.
In the recipe, gelatin powder is used. You use the gelatin to thicken the liquid. Do you have sheets instead of gelatin powder? 1 teaspoon gelatin powder corresponds to 4 grams of gelatin. A sheet in Holland weighs about 2 grams, so you have to use 2 sheets of gelatin instead of 1 teaspoon gelatin powder. You can calculate what every sheet weighs by taking the total weight of gelatin sheets (described on the package) and divide it by the total amount of sheets. In this recipe, you have ¾ teaspoon gelatine powder, whereas every sheet weighs 2 grams so you will need 1 and ½ sheets of gelatin. Allow the sheets to soak in cold water for 5 minutes before you dissolve them in the hot custard (the moment just before you sieve the custard. Stir well so that the gelatin is dissolved before you start to sieve the custard!
The cream mixture is mixed with a spatula through the custard (and not with a whisk or mixer). You want to preserve as much air as possible in the cream. Mixing with a mixer or whisk would make the cream less airy! Use a plastic film inside the cake tin. This way your cake gets much easier out of the tin and remains as beautiful as you shaped it!