Beat eggs until fluffy. Add the sugar gradually and beat until thick. Dissolve the coffee and cocoa in a bowl with water and vanilla. Pour in into the egg-sugar mixture and beat well until combined. Fold in flour and melted butter.
Pour in a paper-lined pan. Bake at 325°F for 20 minutes. Invert at once on a cake rack and cool completely.
Split cake and sprinkle with 1 tbsp. creme de cacao. Fill and frost with Mocha Icing.
Moca icing: Beat egg yolks slightly. Blend sugar and milk. Cook over low fire, stirring constantly til mixture coats the spoon. Cool. Cream butter and add cold egg yolk mixture gradually. Blend in cacao and instant coffee.
NOTE: If you wish to tint icing for decoration, remove enough icing before adding coffee infusion.
