Preheat oven to 335°F / 170°C and prepare two 9-inch cake pans.
Put all ingredients (except hot water and espresso powder) in a large bowl and mix together until totally incorporated and free from lumps. I use a stand mixer but you can also do this by hand.
Dissolve the espresso powder in the hot water and then pour it into the chocolate cake batter. Mix until combined.
Divide cake batter evenly between cake pans and bake for 25 - 35 minutes, or until a skewer comes out clean. Bake time may vary depending on your oven so always test with a skewer before taking out of the oven.
Whip butter with a whisk attachment for 5 minutes until pale and creamy.
Add condensed milk slowly and gradually, a little at a time, whipping in inbetween additions.
Dissolve espresso powder in 1 tablespoon of hot water, add to the frosting along with salt and vanilla. Whip until combined.
Whip for a further minute, frosting should be silky smooth and stiff enough to pipe. If not, see notes.
Notes
Toggle the Cups/Grams button to switch between the different measurements. If your Russian buttercream isn't thick and glossy, try one of these things:If it is thin and soupy, the butter was probably too warm - place buttercream in the fridge for 10 minutes then try whipping up again. If it has curdled - the butter was likely too cold - try heating the buttercream very gently over a bain-marie until it just starts to sweat, then whip up for 5 minutes and it should be saved.