Preheat the oven to 175 ℃ (340 ℉). Put in the bowl of the mixer or a metal mixing bowl, if you're doing it by hand, the flour, sugar, baking soda, baking powder and cocoa, all sifted, and mix thoroughly. Add the salt.

Add the milk, egg yolks, vanilla paste and sour cream. Mix slowly at first until incorporated then beat fast for at least 2 minutes. Then add then melted chocolate and beat again on medium speed until chocolate is fully incorporated.

Pour into a prepared 23cm springform tin and bake on the middle shelf of the oven for 40-45 minutes or until a skewer comes out clean. Cool for 5 minutes before releasing from the springform tin. Place on wire rack to cool completely

For the Frosting, mix together vigorously cocoa powder, powdered sugar, cubed, softened butter and the hot water until a paste forms. Mix in the melted chocolate. Leave at room temperature for an hour before spreading on the top and around the of the cake.