Preheat the oven to 180° C. Cover with butter paper and grease a rectangular baking pan of approximately 43 cm x 30 cm.
In a large bowl, sift the flour with the salt. Add the sugar and mix with a spoon.
Separately, in a medium saucepan, add the butter and bring to medium heat. Let it cook for 1 minute until it melts, then add the cocoa powder with the instant coffee and mix with a spoon to integrate.
Add the boiling water to the pot, mix with the same spoon and cook for 1 more minute until a smooth and homogeneous mixture is obtained.
Remove the pot from the heat, pour the mixture over the bowl with the flour and mix with a balloon whisk until everything is perfectly integrated. Let cool for 10 minutes.
Add the buttermilk with the baking soda, eggs and vanilla extract to the same bowl with the chocolate and flour mixture, and beat for about 30 seconds until smooth and homogeneous.
Pour this mixture into the mold and spread over the entire surface with a spatula. Bake at 180° C for 20 minutes.
For the frosting:
Meanwhile, to make the frosting place the butter in a medium saucepan and bring to medium heat. Cook for 1 minute until melted, add the cocoa powder with the salt, mix and cook for 1 minute more.
Remove the pot from the heat, add the milk with the vanilla extract and mix everything very well. Add the powdered sugar and mix carefully until the frosting is very smooth and light.
Once ready, remove the cake from the oven and let it cool for 5 minutes. When it is still warm and without taking it out of the mold, add the frosting on top and spread it all over the surface with a spatula.
Let the cake stand for 10 minutes and cut into squares to serve.