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1
Preheat oven to 350, and generously butter a 9-inch springform pan.
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2
Sift cake flour, baking flour, and salt together in a medium sized bowl and set aside.
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3
Beat the egg yolks together in a large bowl on high for 3 minutes.
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4
Then with mixer still running, gradually add the 1/3 C sugar and continue beating until thick, light yellow ribbons form in the bowl, about 5 minutes.
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5
Beat in the vanilla and lemon extracts.
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6
Sift the flour mixture over the batter and stir in by hand until no more white flecks appear.
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7
Then blend in butter.
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8
In a clean bowl, using clean dry beaters, beat the egg whites and cream of tartar together on high until frothy.
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9
Gradually add the remaining 2 tablespoons of 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).
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10
Gently spoon the batter into the pan.
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11
Bake the cake until just the center of cake springs back when lightly touched, only about 10 minutes.
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12
Cake should not be brown on top.
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13
Let cool in pan on wire rack while you continue with the cheesecake filling.
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14
For Cheesecake filling: Place 8 oz cream cheese, 1/3 C sugar and the corn starch in a large mixing bowl.
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15
Beat with electric mixer until creamy, about 3 minutes, then beat in remaining cream cheese, 8 oz at a time.
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16
Increase mixer speed to high and beat in remaining 1 1/3 C sugar, then beat in the vanilla.
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17
Blend in the eggs, beating the batter well after each one.
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18
Blend in key lime juice.
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19
Blend in the heavy cream by hand just until blended.
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20
Be careful not to overmix the batter.
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21
Gently spoon the filing on top of the baked sponge cake layer.
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22
Place the springform pan in a large, shallow pan containing hot water that comes about 1 inch up the side of the pan.
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23
Bake the cheesecake until the center barely jiggles about 90 minutes.
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24
Cool the cake on a wire rack for an hour then cover in plastic wrap and refridgerate until completely cold.
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25
*omit the Key Lime juice for plain cheesecake.
