Verdens Beste Kake (The World's Best Cake)

For the pastry cream:
2 cups whole milk
1 vanilla bean, seeds scraped
1 whole egg plus 1 egg yolk
½ cup superfine sugar
¼ cup cornstarch
½ tsp. salt
2 Tbsp. butter
For the cake:
1¼ sticks butter
⅔ cup granulated sugar
5 egg yolks
1 cup plus 2 Tbsp. all-purpose flour or cake flour
1 tsp. baking powder
1 tsp. vanilla extract, or the seeds from 1 vanilla bean
⅓ cup plus 1 Tbsp. whole milk
For the filling:
⅔ cup whipping cream
½ portion of pastry cream
For the meringue filling:
5 egg whites
A pinch of cream of tartar
1¼ cups granulated sugar
⅔ cup slivered almonds

Make the pastry cream:
In a saucepan, heat the milk with the scraped out seeds from the vanilla bean.
In a separate bowl, whisk together the eggs and sugar and add the cornstarch.
When the milk has just reached boiling point, take off the heat and pour one third into the egg mixture while whisking continuously.
Once whisked through, pour the egg mixture back into the remaining hot milk. Return to the stove and bring to the boil, carefully. Whisk continuously as the mixture thickens, for just under a minute, then remove from the heat and stir in the salt and butter.
Pour into a cold bowl and place a sheet of baking parchment on top to prevent the cream from forming a crust as it cools. The mixture will keep well in the refrigerator for a few days.
Make the cake:
Preheat the oven to 325°F. Prepare a 9 by 13-inch rectangular cake pan, greased and lined with baking parchment
In a stand mixer (or using a hand-held electric whisk) cream together the butter and sugar until pale and light. Add the egg yolks one at a time, beating to ensure everything is well incorporated. Sift in the all-purpose or cake flour, baking powder and vanilla and fold in.
Lastly, add the whole milk and fold again until fully combined. Spoon the mixture into the prepared pan and spread out evenly and set aside for a moment.
Next make the meringue topping. Using a completely clean bowl, whisk the egg whites with the cream of tartar until soft peaks form. Add the sugar very slowly, bit by bit, beating on high speed until stiff peaks form (about 5 minutes). Spread the meringue mixture on top of the cake mixture. Scatter the slivered almonds on top.
Bake in the preheated oven for 35–40 minutes or until a skewer inserted into the middle comes out clean and the meringue is firm. Leave to cool for a few minutes in the pan then turn out carefully, so the meringue is still on top. Leave to cool completely.
Whip the cream until stiff and fold together with the pastry cream.
To assemble, cut the cake into two halves. On one half, spread the pastry cream mixture, then carefully layer the other half on top. Leave to set in the refrigerator for an hour before serving. The meringue will stay mallowy and the base soft.