Preheat the oven to 355°F (180°C). Butter and flour an 8-inch (20 cm) round cake pan and line the bottom with parchment paper round. Sift flour, salt, and baking powder twice in a separate bowl.

To make the cake batter, place the softened butter, granulated, and vanilla sugar in a large bowl or the bowl of your stand mixer. Beat with an electric hand mixer for 10-15 minutes until whitish.

Switch the mixer to low speed and continue whisking. Add one egg at a time and beat until the egg is mixed up. Add the next one only when the previous one has been used up.

Pro tip: You may get a slightly grainy mixture. Avoid increasing the mixer speed in an attempt to resolve this issue. Adding flour will fix any slight graininess in the mixture.

Remove the mixer's bowl from the mixer and add the sifted dry ingredients one tablespoon at a time. Use a rubber spatula to combine; gently mix with large movements from the bottom up. Continue until the flour mixture is mixed into the batter.

Pour it into the prepared pan and smooth with a spatula. Bake for 40-45 minutes until a toothpick inserted into the cake's center comes out dry.

Remove the cake from the oven and flip it on a cooling rack lined with parchment. Allow it to rest for 10-15 minutes, avoiding excessive condensation on the paper. Then turn the cake over again, and let it cool down completely.

To make chocolate shavings use a short knife or a vegetable peeler to shave a chocolate bar. Alternatively, make chocolate curls out of melted chocolate.

To make the chocolate frosting, chop the chocolate into small pieces. Pour heavy cream into a small saucepan and warm it up. With the first bubbles, add hot cream to the chopped chocolate and let it stay for 1 minute.

Use a wooden spatula, and mix the chocolate mixture until the chocolate is melted. Add softened butter and mix until smooth. 

Let the chocolate frosting stay at room temperature to thicken, stirring occasionally. It should reach a spreadable consistency (it takes about 30-40 minutes).

To assemble the cake, use a serrated knife or a cake leveler to cut the cake into three cake layers 1 cm thick.

Place the first cake layer on a cake board, and spread 1/4 of the frosting. Then add the second layer and cover it with another 1/4 of the frosting. Finish by placing the third layer.

Cover the entire cake (the top and sides) with the chocolate frosting using an offset spatula. Sprinkle the top of the cake with chocolate shavings.