Tangerine Sponge Cake with Citrus Syrup and Butter Cream

When a festive cake is needed, sponge cakes come to mind. Though they are made with very few ingredients, the way of preparing the batter is what gives them their fluffy spongy texture. 
My usual go-to recipe, when it comes to sponge cakes, is my orange sponge cake, with maple-rum butter cream, which is also the base for versions I’ve created over the years. (Click HERE for the recipes).
This time, I’ve decided to prepare the cake with tangerine juice and zest, instead of orange juice, and to also add a citrus syrup to it, on top of the butter cream.
The result was highly enjoyable; the light cake had wonderful citrus aroma, which was enhanced further by the addition of the syrup. The butter cream gave a smooth and pleasing coating to both. The cake is perfect for serving at any type of celebration. Try it and enjoy.

Notes:
* For best results, all the ingredients should be at room temperature.
* If you want to end up with a fluffy cake, make sure not to skip the sifting stage.
* Any trace of fat in the egg whites or the bowl will prevent proper whipping. Note that egg yolks are fatty, so separate the eggs carefully.
* Egg whites whip better with a bit of acidity, which is why we add the vinegar. The vinegar will not affect the flavor, so no need to hesitate about using it.

Prep time: 30 minutes
Baking time: 40 minutes
Cooking time for the syrup: 10 minutes
Makes: 12-16 portions

Ingredients:
For the cake:
2 cups flour
2 tsp baking powder
¼ tsp salt
3-4 medium tangerines, washed and dried
6 L eggs, at room temperature
1½ cups sugar
¼ cup oil
½ tsp vanilla
½ tsp vinegar
For the butter rum cream:
2 cups powdered sugar
¼ tsp salt
2 sticks (230 grams) butter, very soft
3 Tbs milk
1 tsp vanilla
1 Tbs maple syrup
1 Tbs spiced rum
For the citrus syrup:
½ cup honey (preferably orange blossom)
¾ cup water
1 tangerine, washed, sliced thin
Zest from ¼ lemon
1 Tbs spiced rum

1. The cake: preheat the oven to 350F (175C). Oil and lightly dust with flour a 10” (26 cm) deep cake pan. Turn upside-down, and tap over, to get rid of excess flour. Keep in the fridge until using.
2. Sift together the flour, baking powder and salt. Set aside. Zest the tangerines, measure 2 Tbs and set aside. Juice them and measure 1 cup juice. Set aside.
3. Separate the eggs: place the egg whites in a medium bowl, and the egg yolks in a large mixing bowl.
4. Add the oil to the egg yolks and mix well. Add the tangerine juice and zest, vanilla and ¼ cup of the sugar, and whisk together.
5. Add the vinegar to the egg whites and start whipping on medium-low speed. When large bubbles start to form, increase the speed to medium and start adding the sugar slowly. Increase the speed to medium-high and continue whipping until a firm and shiny meringue is formed.
6. Add a small amount of the meringue to the bowl with the egg yolks, and whisk well (this will fluff the mixture). Now add about half of the rest of the meringue, and this time, whisk gently, so that the meringue will not break.
7. Add the flour mixture and fold gently with the whisk. Change the whisk to a spatula, add the rest of the meringue and mix gently, to form a smooth batter. Do not over mix.
8. Pour the batter into the pan. Bake for 40 minutes, until the cake is golden, pulls away from the sides of the pan, and when you press the center with your finger, it bounces back.
9. Transfer the cake to a rack and let cool for 3 minutes in the pan. Turn the cake upside-down onto the rack, and let cool completely.
10. The butter cream: place the butter in a medium bowl, and whisk on medium speed until it is fluffy and pale. Scrape the sides at least once.
11. Add the powdered sugar and salt through a sieve, and cream, on very low speed, to incorporate, while scraping the sides from time to time.
12. Add the milk, maple syrup and rum. Increase the speed to medium-high, and keep creaming for about 5 minutes. Scrape the sides and cream for 4-5 minutes longer, until the cream is pale and smooth. Keep at room temperature.
13. The syrup: mix all the ingredients in a small pot. Bring to a boil, lower the heat to low, and cook for 10 minutes. Let cool to room temperature.
14. Assembly: drizzle a bit of the syrup on top of the cake. Let soak for a couple of minutes, before spreading a thin layer of the butter cream over the top and sides of the cake, with a large spatula. Transfer to the freezer for 5 minutes, to let the first layer harden. (This step helps spread the rest of the cream more evenly and without crumbs.)
15. Spread the rest of the cream evenly all over the cake. Transfer the cake to the fridge for at least an hour, to let the butter cream set. Carefully, using a large flat spatula, transfer the cake to a serving plate.
16. Serve with sliced tangerines, and the remaining syrup, on the side.