For the cake, preheat the oven to 350 °F. Grease 2 8-inch cake pans and line the bottoms with parchment. Dust the sides of the pans with flour, tapping out any excess flour.
Sift the flour, sugar, cocoa powder, baking soda and salt into a large mixing bowl or into the bowl of a stand mixer fitted with the paddle attachment. Add the butter cutting it in (if in a large bowl, use electric beaters) until the mixture is a fine crumble (like the texture of fine breadcrumbs) and no large pieces of butter are visible.
Stir the hot coffee, milk and vanilla together and add it all at once to the flour mixture, blending until smooth. Break the eggs into a small dish and stir them with a fork and then add them to the batter, again blending on medium speed just until smooth (the batter will be very fluid). Divide the batter evenly between the prepared pans and tap them on the counter to eliminate any bubbles.
Bake the cakes for about 30 minutes, until a tester inserted in the centre of the cake comes out clean. Cool the cakes for 20 minutes in their pans, then tip out onto a cooling rack to cool completely to room temperature. The cakes can be baked a day ahead, wrapped and stored at room temperature before frosting.
For the frosting, place the chocolate and cream in a metal bowl and place it over a pot of barely simmering water. Stir the chocolate and cream occasionally and gently until all the chocolate has melted and the mixture is smooth. Remove the bowl from the heat and whisk in the sour cream, vanilla and salt. Let the mixture cool completely to room temperature, stir occasionally (you can chill the mixture for just a few minutes to start it setting, then let it continue to cool naturally at room temperature). Once cooled to a thicker consistency, whisk by hand to thicken just until spreadable consistency (do not whip vigorously or with electric beaters).
Place the first cake layers on your serving plate and, if needed, trim the cake to level it. Spread a generous layer of the frosting over the top of the cake. Trim the second cake, if needed and place this on top of the first cake layer. Spread frosting over the top and then the sides of the cake, using swirling motion with your spatula to create a nice design. Chill the cake for at least 2 hours, or until ready to serve.
The cake will keep, refrigerated, for up to 3 days.