Maria Aleksandrovna tweaked the Invisible Apple Cake recipe so you can make a vanilla sauce to go with it without having any egg whites left over. If you're not making the sauce, just use whole eggs.
Recipe:
1.2 kg tart apples, from the Vasula stall
3 eggs, or 4 egg whites + 1 whole egg (if making the sauce)
1.25 dl (125 g) sugar
1.5 dl milk from the Pajumäe stall
40 g butter
1.5 dl (90 g) flour
1.5 tsp baking powder
Vanilla sugar or even cardamom instead
Instructions:
1. Melt the butter.
2. Whisk the eggs with the sugar, then add the melted butter and milk.
3. In another bowl, mix the flour, baking powder, and vanilla, then sift into the egg mixture and stir.
4. Wash, core, and peel the apples, then cut into quarters.
5. Slice the apples very thinly, about 1 mm thick, either by hand or with a food processor.
6. Fold the apple slices into the batter. Yes, it's just a thin layer of batter—that's exactly how it should be!
7. Use a solid-bottom cake pan, or line a springform pan with parchment paper. The batter is quite runny and will otherwise find a way to leak onto your oven floor. 
8. Pour the batter into the pan and gently smooth the top.
9. Bake at 180°C for 60–70 minutes. If the top apples start to brown too much, you can cover the pan with foil toward the end of baking.
Vanilla sauce:
4 egg yolks (left over from making the cake)
5 dl (500 ml) milk
50 g sugar
Vanilla or vanilla sugar
2–3 tsp cornstarch – optional
Instructions:
1. Whisk the egg yolks with the sugar until light and fluffy. Add the cornstarch if using.
2. Pour 1/3 of the hot milk into the egg foam, whisking or mixing continuously.
3. Pour all of the egg mixture into the pot and mix with an electric hand mixer so the sauce doesn't scorch while heating. The sauce should reach around 75–85°C; the main thing is not to let it boil. Using an electric mixer makes the sauce wonderfully light and frothy. If you prefer a smoother, un-aerated sauce, just whisk by hand.
4. Pour the sauce over the cake and enjoy!
