Grease 2, 9-inch (23cm) or 8-inch (20cm) round baking pans and line base & side with parchment paper. Preheat the oven to 350°F (180°C).
Combine the coffee, sugar, butter, cocoa & baking soda in a large saucepan or pot with a high side – the mixture will bubble up. Stir over medium heat until the butter is melted & sugar is dissolved. Increase the heat to medium-high & bring to a boil, taking care the mixture doesn’t boil over. Remove from the heat; let cool. Pour into a large bowl (or you can mix it all up on one saucepan if it’s big enough to allow stirring & whisking).
Once the mixture is cooled, add the sour cream & eggs & whisk until well combined. Add the flour & salt & whisk until no lumps are remaining. Pour the mixture into the prepared pans & gently tap the pan on the counter to remove some of the large air bubbles. Bake for 45 to 50 minutes or until a skewer inserted into the center comes out clean. Let cake stand for 10 minutes before removing to a wire rack to cool completely.
To make the icing:
Combine the butter, coffee & sugar in a medium saucepan. Stir over medium heat until the sugar is dissolved. Sift confectioner’s sugar, cocoa & salt into the butter mixture. Use a whisk to remove any lumps. Pour into a bowl; cover & chill for 1 to 2 hours, stirring occasionally, until the mixture is thick & spreadable. If you make this ahead of time, leave out of the fridge before you spread it on the cake, it needs to be the consistency of peanut butter.
Cut each cake horizontally in half so you have 4 cakes, or you can do just two layers – much easier. Place one piece of cake on a cake stand or serving platter. Spread the cold cake with the icing. Add another layer of cake & spread with some of the icing. Repeat the layer until the four layers are stacked. Spread the remaining icing over the top & side of the cake. If using chocolate sprinkles, carefully press the sprinkles into the side of the cake. Cake can be made a day ahead & stored well-covered in the fridge. Remove from the fridge 30 minutes before serving. Serves 10.