Preheat oven to 160 C, 325 F. Grease and lightly flour two 8 inch or 20 cm loose bottomed round cake tins.
Sift flour, cocoa powder, salt and baking powder. In a separate bowl, sift cocoa powder. In a cup, add the coffee granules and semi sweet chocolate bits then add the 5 Tablespoons boiling hot water. Stir until dissolved. Set aside.
Lightly beat eggs and add vanilla in a small bowl. In a mixing bowl, cream butter and sugar till light and fluffy.
Add egg mixture, a little at a time to the batter. Then add half the flour, then milk, then the rest of flour and then the chocolate, coffee and hot water mixture (from the cup). Combine everything.
Transfer the batter to the 2 baking pans and bake for 30 - 35 mins or until skewer comes out clean. Allow the cakes to rest for 2 minutes then remove from pans and place UPSIDE DOWN on a cooling rack. Allow to COMPLETELY cool before frosting.
*** Please note, bake times will vary slightly so please check for done - ness after 25 - 30 minutes of baking.
Make the Chocolate Filling and Topping:
Place both chocolates in a heatproof bowl, melt together over a saucepan of simmering water. Once melted, remove from heat and stir until smooth. Then add the sour cream and whisk using an electric whisk until it becomes thick.
Place one of the cakes on to the serving dish then spread some of the chocolate filling evenly over the cake. Place the second cake on top of the filling. Then spread the remaining over the top of the cake and decorate with some chocolate shavings.
***If the weather is hot, pop the filling in the fridge for 10 minutes to cool and thicken.