Sift cocoa into a medium bowl, add semisweet chocolate and instant coffee. whisk in boiling water and set aside to cool.
Whisk buttermilk into reserved cocoa mixture.
Sift together flour, baking soda and salt.
In a large bowl, cream butter and gradually beat in sugar until light and fluffy, 3 to 4 minutes, scraping down sides twice.
Beat in vanilla. Drizzle in eggs, a little at a time, beating between each addition until the batter is no longer slick.
Alternately add flour and cocoa mixtures to the batter, a little of each at a time, starting and ending with flour mixture.
Divide batter evenly among two greased pans. Bake at 360° F until a cake tester inserted into center of each layer comes out clean, about 40 min.
For the frosting:
Hydrate the unflavored gelatin in the 140 ml of cold water and set aside.
Melt in a double boiler both chocolates, then, tun off the stove and add butter and the tbsp of cold water.
In a pan, boil the cream with the sugar and the sifted cocoa powder. Stir in the already hydrated gelatin and make sure to keep stiring otherwise it will burn and form bumps.
Mix this with the previous chocolate mixture and let it cool completly.
To assemble:
Place one cake layer on a serving platter and spread just a part of the chocolate frosting over the top.
Add the second cake layer and spread with the rest of the frosting.