Grate the zest from the orange and rub it with sugar. So that the sugar becomes moist and absorbs the flavor of the zest.
Next, in the bowl of a mixer, mix all the dry ingredients (flour, sugar, cocoa, baking powder, salt, soda). In a separate bowl, mix the liquid ingredients, except milk (eggs, melted butter, yogurt, 30 g of orange juice).
At low speed, add the liquid ingredients. Then add the milk. At higher speeds, whisk until smooth.
Bake in a mold with a diameter of 16 cm at a temperature of 160 degrees Celsius for 40 minutes.
The finished sponge cake can be used immediately, but it is advisable to wrap it with a film and put it in the refrigerator overnight.
Cut the biscuit into 3 pieces.