Fruity triangles with a filling of yoghurt and mandarins

This cake is a nice refreshing summer cake. The filling contains yoghurt, fruit and some whipped cream making it quite light. Neither the preparation of the sponge cake nor the filling is too difficult, but some practice could be useful (see my comments below).

Baking time: 10 min
Cooling time: at least two hours
Total time: 2.5 h

Ingredients:
baking sheet (40 x 30 cm)
some grease
two sheets of parchment paper, a little bigger than the baking sheet

Sponge cake:
3 eggs
3 tablespoons hot water
50 g potato starch
100 g flour
125 g sugar
1 package vanilla sugar or some vanilla aroma
1 teaspoon baking powder

Filling:
2 tablespoons lemon juice
200 g cold whipping cream
1 package gelatine powder
500 g quark or plain Greek yoghurt (fat free, Fage or Chobani brand)
2 cans mandarin oranges
1 package vanilla sugar or some vanilla aroma
125 g sugar
some grated lemon zest

Decoration: some powdered sugar

Preparation:
1. Grease baking sheet and line with parchment paper. Fold parchment paper to form edges around baking sheet. Preheat oven to 200°C (350F).

2. Sponge cake: Mix eggs and hot water with a hand mixer at highest setting until foamy. Add in sugar and vanilla sugar or vanilla aroma and mix for an additional 2 min. In a separate bowl mix flour, starch and baking powder. Add flour mixture to eggs and mix in for about 1 min on low speed setting. Pour batter onto parchment paper on baking sheet, flatten evenly, and bake for about 10 min until golden brown.

3. Prepare a second piece of parchment paper and drizzle sugar on it. This will prevent the sponge cake from sticking to the parchment paper after baking. Directly after baking, carefully remove the baked parchment paper from the edges of the baked sponge cake, then put the sponge cake upside down onto the parchment paper drizzled with sugar. Carefully remove baked parchment paper from the back of the sponge cake. Let sponge cake cool fully, the cut in half to make two plates of about the same size (30 x 20 cm).

3. Filling: Drain canned mandarin oranges with a colander, collect syrup. Mix gelatine powder with 3 tablespoons of the mandarin syrup in a pot. Let sit for 5 min. Slowly heat mixture on the stove to dissolve gelatine.
In a bowl mix Greek yoghurt with sugar, vanilla sugar or vanilla aroma, lemon juice and lemon zest. Don't use a hand mixer for this! Add four tablespoons of the yoghurt mixture with a whisk to the dissolved gelatine, then add gelatine into yoghurt mixture using a whisk, not a hand mixer.
Whisk whipping cream until stiff. Carefully fold first whipping cream, then mandarin oranges into yoghurt mixture.

Add filling on top of first half of sponge cake, add second half of sponge cake on top and push down a little to make it stick. Cool cake for at least two hours before serving. Cut into squares or triangles and serve.

4. Decoration: Drizzle powdered sugar on top, and cut cake into pieces.

Comments:
1. Other canned fruits can also be used, e.g. canned peaches or pineapple. Do not use fresh pineapple. Fresh pineapple contains an enzyme that cleaves the gelatin, so the filling won't become stiff!
2. I used a hand mixer to make the yoghurt filling - big mistake! It turns the Greek yoghurt into a liquid, the gelatin cannot really do its job, making it difficult to assemble the cake. To rescue it, I first pre-cooled the yoghurt mixture in the refrigerator for about 20 min. To assemble the cake I lined a Pyrex form with parchment paper, added the bottom sponge cake layer, then added one half of the filling, cooled it for about 30 min in the refrigerator, added the second half of the filling, and cooled the cake overnight. I did not add the top layer yet, which made my version a little dry. If the top layer is cooled with the cake, it soaks up some of the moisture from the yoghurt making it softer and less dry.
3. Cutting the cake can be a little tricky because the filling can squeeze out from the sides when cutting. To prevent this from happening the top layer can be cut on a board into the desired serving size, and re-assembled on top of the filling. Using the pre-cut top layer, the bottom and filling can be easily cut along the top layer.
4. As I had mentioned in comment 2 I didn't put the top layer on during the chilling time, which made it a little dry. I had left one piece with the top in the refrigerator for another night, and it was perfect. Now both layers, top and bottom, were moist, but not soggy! In conclusion, prepare this cake at least a day or 1.5 days ahead, so that it will be just perfect when it comes to serving it.