Place egg yolks in a large mixing bowl; beat at high speed of an electric mixer 6 minutes or until thick and lemon colored. Add lemon rind and juice; beat at high speed an additional 4 minutes.
Gradually add sugar, 2 tablespoons at a time, beating 5 minutes or until smooth.
Combine flour, baking powder, and salt; add to yolk mixture alternately with milk, beginning and ending with flour mixture. Mix well after each addition.
Beat egg whites (at room temperature) until soft peaks form. Gently fold 1 cup of egg whites into yolk mixture. Gently fold yolk mixture into remaining egg whites.
Pour batter into an ungreased 1 0 - inch tube pan, spreading batter evenly with a spatula. Bake at 375° for 50 minutes or until cake springs back when lightly touched. Remove cake from oven, and invert pan. Cool cake 40 minutes, and remove from pan.
Press almonds into sides and top of cake; place cake in a 12- cup trifle bowl. Immediately before serving, pour 1 cup sherry evenly over cake. Spoon Boiled Custard over cake and into hole in center of the cake.
Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form. Fold in remaining sherry.
Top cake with sweetened whipped cream, and garnish with maraschino cherries and toasted almonds. Spoon into serving bowls, and serve immediately.
Note: Slivered toasted almonds may be used in place of blanched almonds.