Preheat the oven to 350 F and generously butter a 9-inch springform pan. Sift the cake flour, baking powder, and salt together in a medium-sized bowl and set aside.
Beat the egg yolks together in a large bowl with an electric mixer on high for 3 minutes. Then, with the mixer still running, gradually add the 1/3 cup of sugar and continue beating until thick light-yellow ribbons form in the bowl, about 5 minutes more. Beat in the vanilla and lemon extracts.
Sift the flour mixture over the batter and stir it in by hand until no more white flecks appear. Then blend in the butter.
In a clean bowl, using clean dry beaters, beat the egg whites and cream of tartar together on high until frothy. Gradually add the remaining 2 tablespoons sugar and continue beating until stiff peaks form (the whites should stand up in stiff peaks, but not be dry).
Stir about 1/3 cup of the whites into the batter, then gently fold in the remaining whites (don't worry if a few white specks remain).
Gently spoon the batter into the pan. Bake the cake just until the center of the cake springs back when lightly touched, only about 10 minutes (watch carefully!). Let the cake cool in the pan on a wire rack while you continue making the cheesecake filling. Do not remove the cake from the pan.
While the sponge cake is baking, make the lemon custard. With clean, dry beaters, beat the egg yolks with mixer on high about 5 minutes until thick & light golden.
Combine the sugar, cornstarch, & salt together in a large saucepan. Gradually whisk in the water until smooth & all the cornstarch lumps have disappeared.
Cook the custard over medium heat, stirring constantly, until it comes to a boil, turns clear, & thickens, then immediately remove from heat.
Spoon about a cup of the hot cornstarch mixture into the egg yolks, then return it all to the saucepan. Stir over low heat just until the mixture begins to cook & bubble around the edge of the pan ( do not overcook at this stage, as too much heat can cause the custard to separate).
Remove from heat & stir in the lemon rind & juice, butter, extracts, & a couple of drops of food coloring to tint to light yellow, if you wish. Transfer to a medium- size bowl, place a piece of plastic wrap directly on its surface to prevent a skin from forming, & refrigerate until cold, at least 45 minutes or over night (don't freeze).
While the cake cools, make the cream cheese filling: Place one 8-ounce package of the cream cheese, 1/3 cup of the sugar, and the cornstarch in a large bowl. Beat with an electric mixer on low until creamy, about 3 minutes, then beat in the remaining 3 packages of cream cheese.
Increase the mixer speed to high and beat in the remaining 1 1/2 cups of the sugar, then beat in the vanilla and heavy cream. Blend in the eggs, one at a time, beating the bating only until completely blended (just like they do at Junior's). Be careful not to over-mix the batter.
Gently spoon the cheese filling into a 9-inch springform pan. Place the springform pan in a large shallow pan containing hot water that comes about 1 inch up the sides of the pan. Bake the cheesecake until the center barely jiggles when you shake the pan, about 1 hour.
Cool the cake on a wire rack until completely cooled. Wrap tightly and freeze at least 2 hours.
Let the sponge cake cool 2 hours then wrap in plastic wrap & refrigerate till until ready to assemble cake.
When ready to assemble, remove cheesecake from freezer & let stand at room temperature while you make the frosting. Place the gelatin in a heat proof measuring cup, stir in cold water, & let stand until it swells & thickens. Cook in the microwave on high for about 30 seconds or over a pan of simmering water for about 1 minute, until clear & completely melted. In a medium- size bowl with clean, dry beaters, whip the cream with the mixer on high until it thickens & soft peaks just begin to form. With the mixer still running, add the sugar & beat just until the cream stands up in peaks ( Don't over-mix or the cream will curdle). Beat in the vanilla. Add the melted gelatin all at once & beat until thoroughly incorporated. Refrigerate the cream in 2 bowls: 2 cups in small bowl for piping decorations on the top of the cake & the remainder in a second bowl for frosting the cake.
Remove the lemon custard from refrigerator, transfer 2 tablespoons to a small bowl, & return this to the refrigerator ( to use later to decorate the center of the cake). Place the sponge cake, top side down, on a cake plate. Spread with a thin layer of whipped cream from large bowl, sprinkle with about 2/3 cup of the coconut, & gently spread with half of the lemon custard. release & remove ring of the springform pan, then remove the frozen cheesecake from the bottom pan. ( if you have trouble removing the metal bottom warm the bottom of the pan on a low burner for 30 seconds, just long enough to melt the butter which was used to grease the pan) place top side down on the custard. Top cheese cake with some more whipped cream, sprinkle with 2/3 cup coconut, & spread with the remaining custard in the large bowl.
To decorate, frost top & sides of the cake with the remaining whipped cream in the large bowl. Using fingers, gently pat coconut on the side of cake & sprinkle the rest on top. fit a pastry bag with a medium closed star tip ( # 32) & fill with the cream from the small bowl. Pipe shells or rosettes in the center. Fill the circle with the 2 tablespoons reserved lemon custard.
Refrigerate the cake for at least 2 hours to allow the cheesecake to thaw enough to slice easily. Use a sharp straight edge knife, not a serrated one, to cut it. Cover any leftover cake with plastic wrap & refrigerate, or wrap & freeze for up to 1 month.