Below is the video of the sponge cake recipe I have been using for many months now:
While your sponge cake dough is inside the oven, you can begin making the sweet and sour chocolate topping. Here are the ingredients and the instructions you need for this delicious chocolate topping:

• 200 grams dark chocolate
• 50 ml sour cream
• 30 ml powdered sugar or confectioner's sugar

Break the chocolate bar into pieces. Put together the chocolate bits and sour cream in a little pan. Heat the mixture in very low heat. Keep on mixing until the chocolate is fully melted. When done, take the pan off the heat and mix in the sifted confectioner's sugar. Blend well. If you want the topping to be sweeter, just add some more confectioner's sugar little by little until you find the taste satisfying for you. Spread the chocolate topping on your newly baked sponge cake. Allow to cool inside your fridge.

Enjoy!