If not having buttermilk, mix together milk and vinegar in a cup and leave for 5 minutes without stirring.
For the sponge, combine the dry ingredients. Sift and combine flour and cocoa powder, add baking powder, baking soda and salt.
Separate yolks from egg whites.
Start beating yolks with sugars until fluffy. Then add the liquids: buttermilk, oil and vanilla extract. Keep mixing until combined. Then gently incorporate the dry ingredients.
When previous steps are done, gently incorporate egg whites that you have beaten separately. Do it in two batches by using a silicone spatula.
Grease a round 20 cm (10 inch) baking mould with butter and put parchment paper on the bottom.
Preheat you oven to 180 °C or 350 F. Transfer the batter into the mould and bake until a toothpick comes out clean. It will take about 20 - 25 minutes. Do not over-bake because then your sponge will be dry.
For the frosting, start off by melting the chocolate. I prefer doing it with bain marie (a water bath or double boiler), but you can use microwave if you prefer.
When melted, leave your chocolate on the counter to cill down slightly.
In the meantime, mix mascarpone until fluffy. Second, add chocolate and keep mixing. Now, add whipping cream and mix until batter becomes silky. Watch out not to over-mix and let you frosting go lumpy. Add sugar if wanted.
You can put your frosting into the fridge until sponge cools down to room temperature.
Once ready, cover your soft sponge with creamy frosting. And that is it. You can dust it with some cocoa powder or sprinkle with some chocolate shavings.
Let the whole cake set in the fridge for a couple of hours, preferably over night.