1. Preheat oven to 350'F.
2. Grease and flour 2 9" layer cake pans.
3. Add soda to buttermilk, stir to blend, and set aside.
4. Cream butter with sugar until light and fluffy. Add egg yolks one at a time and beat well after each addition.
5. Sift flour with salt and baking powder onto waxed paper. Add flour mixture alternately with buttermilk to butter a little at a time and blend well after each addition. Stir in lemon rind.
6. In a separate bowl beat egg whites until stiff, then fold into batter. Pour into prepared pans and bake in preheated oven for about 30 minutes. Remove from pans onto cake rack until cool. Put layers together with Lemon-Coconut Filling, frost with Coconut Frosting, and sprinkle with coconut.
*** Lemon-Coconut Filling *** Mix together sugar and cornstarch in a saucepan. Add boiling water and lemon juice; stir in coconut. Bring to a full boil over high heat, stirring frequently. Reduce heat slightly and let boil, stirring constantly, for 1 minute. Remove from heat and stir in butter. Cool to room temperature. Stir; spread between cakes layers.
*** Coconut Frosting *** 1. Combine all ingredients except coconut in top of double boiler. Beat 1 minute to blend, then place over boiling water and beat with rotary hand or electric beater until stiff peaks form, stirring mixture up from bottom and sides of pan occasionally.
2. Transfer to a large bowl and beat 1 minute more, or until thick enough to spread.
3. Frost cake, then sprinkle generously with grated coconut.
4. Makes sufficient frosting for sides and top of 2 9" cake layers.