For the cake, preheat the oven to 165C (325F). Butter a 40 x 30 cm (16 x 12 inch) rimmed baking sheet, then line it with baking paper (parchment paper) and press down so that the paper sticks to the baking sheet.
In a medium bowl, combine flour and baking powder.
Attach a stand mixer fitted with the paddle attachment and mix the butter and sugar until light and fluffy. Add the egg yolks, one at a time, and mix until incorporated. Add milk and vanilla and mix until combined. Add flour mixture and mix until incorporated. The dough should be somewhat thick and sticky. Using a rubber spatula, spread the dough as evenly as possible across the prepared baking sheet, keeping a rectangular shape.
For the meringue, beat the egg whites in a stand mixer fitted with the whisk attachment over medium heat until foamy. Gradually add sugar and beat until shiny stiff peaks form. Pour the meringue over the dough on the baking sheet and use a spatula to spread it evenly and lift it up to create a small mound. Scatter sliced almonds on top. Bake for about 30 minutes or until meringue is dry and lightly golden. Set aside to cool.
For the custard, combine sugar and egg yolks in a large bowl. Add cornflour and whisk until thick and pale yellow.
Pour the milk into a small saucepan. Scrape the seeds out of the vanilla bean and add to the milk (or add the vanilla extract). Heat over medium heat until just before boiling, then remove from heat. Remove the vanilla bean and slowly add the milk to the egg yolk mixture, whisking constantly to prevent the eggs from curdling. Return the mixture to the pot and place over medium heat. Cook, stirring constantly, until thickened. Set aside to cool.
For the cream, whip the cream until stiff peaks form. Stir into cooled custard.
To assemble, use the paper to lift the cake off the baking sheet and place, meringue side up, on a large cutting board with the long edges horizontal. Cut the cake in half lengthwise so you have two cakes of the same size. Carefully hold the large tray over the cake with one hand. Place your other hand under the cake and invert it onto the tray, meringue side down. Peel off the paper. Spread the custard cream on top of the cake on the tray. Peel the paper from the other cake and gently place it, meringue side up, on top of the custard cream to form one cake. Chill in the refrigerator for about 1 hour before serving. Store tightly closed in the refrigerator for 3-4 days.
(Reprinted from Norwegian Baking Through the Seasons by Nevada Berg, Penguin Random House, April 2023.)
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