Preheat the oven to 175°C / fan 155°C. Grease and line a 20 cm round springform tin.

In a large bowl, whisk the eggs, sugar, and vanilla until light and slightly thickened.

In a separate bowl, sift together the flour, baking powder, and salt.

Stir the melted butter and filmjölk into the egg mixture.

Gently fold in the flour mixture, followed by the rhubarb.

Pour the batter into the prepared tin and smooth the surface.

Bake in the lower part of the oven for about 30–40 minutes – the sponge should feel set and start to colour but still pale.

When the cake is almost ready, melt the butter for the almond topping. Add the sugar, milk, and flour. Let it just come to a simmer, then remove from the heat. Stir in the flaked almonds and stir well.

Remove the sponge from the oven and gently spread the almond topping over the surface.

Return the cake to the middle of the oven and bake for another 15–20 minutes, until the topping is golden and bubbling.

Let the cake cool in the tin for about 30 minutes before transferring to a wire rack.

Serve in wedges with a generous spoonful of cold vaniljsås poured over the top.

For the vaniljsås, combine the milk, cream, and vanilla pod in a saucepan. Bring just to a simmer, then remove from the heat and let infuse for 10–15 minutes.

In a separate bowl, whisk together the egg yolks, sugar, potato starch, and salt.

Remove the vanilla pods from the milk mixture and bring it back to a simmer.

Gradually whisk the hot milk into the yolk mixture, then return everything to the pan.

Cook over low heat, stirring constantly, until the custard thickens slightly and just starts to simmer.

Strain into a clean bowl, clingfilm to the touch and chill in the fridge until needed – for at least 2 hours and up to 3 days.