Preheat oven to 350º. Grease and lightly flour two 9x1-1/2” round layer cake pans.
In double boiler top, combine chocolate and hot water. Stirring constantly over hot, not boiling water, melt chocolate. Add ½ cup sugar, cook stirring 2 min, to dissolve. Remove from heat and cool to lukewarm.
In a small bowl, sift flour with soda and salt.
In a large bowl, at medium speed, beat butter, until fluffy. Gradually beat in 1-3/4 cup sugar, beating until very light and fluffy, about 5 min. Add vanilla. Add eggs, on at a time, beating well after each addition, scraping down sides of bowl and beaters. At low speed, add the flour mixture alternately with milk, beginning and ending with flour. Neat just to blend. With a scraper, blend in chocolate. Turn into prepared cake pans. Bake 25 min. or until cake springs back when gently pressed with finger. Do not overbake. Cool for 10 min in pans. Loosen sides with a knife or spatula, Remove cakes and cool on wire rack.
In medium bowl, combine cream, sugar, cocoa, salt and coffee. Refrigerate, covered, ½ hour. Beat with rotary beaters until stiff.
With a long, thin-bladed knife, split cake layers in half to make four. Place layer, cut side up, on plate. Spread with 2/3 cup of mocha frosting, repeat twice. Top with last layer, cut side down. Frost top and sides of cake with remaining frosting.
If using the chopped nuts, sprinkle on top.