Combine 1 cup water and raisins in a medium saucepan over medium-high heat. Bring to a boil; reduce heat, cover, and simmer 10 minutes. Drain and reserve water, adding water to make 1/2 cup; cool.
Combine flour and next 4 ingredients in a bowl.
Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, beating well. Add egg, beating just until combined. Add flour mixture alternately with reserved water beginning and ending with flour mixture; beat just until combined. Stir in raisins and walnuts. Spread batter into 2 greased and floured 9-inch round cake pans.
Bake at 375° for 15 to 20 minutes or until a wooden pick inserted in center comes out clean. Run a knife around edge of pans; cool in pans on a wire rack 10 minutes. Remove from pans, and cool completely on a wire rack. Spread Egg Custard Filling over one cake layer; top with remaining cake layer. Frost top and sides of cake with Mocha Frosting. If desired, place chopped nuts around sides of cake, and garnish with chocolate-covered coffee beans. If using flowers, place in florist's water picks, or wrap stems in foil. Cover and chill until serving. Store cake covered and chilled.