Preheat oven to 300 degrees.
Dissolve soda in water and set aside.
Cream together butter and sugar. Beat in eggs. Sift together dry ingredients. Add soda/water mixture to coffee. Alternate adding coffee and dry ingredients to sugar mixture. Beat well.
Pour batter into 3 (8 or 9 inch) round pans. Bake until toothpick inserted in center comes out clean.
Cool 10 minutes and remove cakes from pan to wire rack. Cool completely.

Beat all ingredients until creamy. You may need to add more powdered sugar if filling is too thin or more coffee if filling is too thick.
Spread between layers of cooled cake.