Preheat oven to 350º. Grease two 9 inch round cake pans. Line bottoms with waxed paper. Beat butter until smooth. Gradually add sugar. Cream until light and fluffy. Combine flour, baking powder and salt. Combine milk and water. Alternately add flour mixture and milk mixture to batter, beginning and ending with flour mixture. Beat well after each addition. Add vanilla and almond extracts. Beat egg white until stiff peaks form. Fold into batter. Pour into prepared pans. Bake 20-30 minutes or until top springs back when gently pressed. Cool in pans 10 minutes before inverting onto a rack to cool completely.
For lemon filling:
Mix sugar, cornstarch and salt on top of a double boiler. Slowly stir in the water and lemon juice. Cool about 15 minutes, stirring constantly, until the mixture thickens. Beat egg yolks. Blend about 1 cup of the hot mixture into the egg yolks to heat. Stir yolks into saucepan. Cook, stirring constantly until mixture becomes very thick, about 5 minutes. Cool and chill.
For the frosting:
Combine frosting ingredients in a heat proof medium bowl. Place over boiling water and beat for 5-7 minutes. Remove bowl from heat and continue to beat until frosting reaches spreading consistency.
Place one layer on a serving platter. Spread with half the lemon filling. Top with second layer. Combine remaining lemon filling with 1 cup seven minute frosting. Spread on top of cake. Frost sides with seven minute frosting.
Place one layer on a serving platter