Preheat the oven to 325 and grease a 9 inch spring form.
In a small bowl, combine graham cracker crumbs, melted butter, vanilla extract, key lime zest and brown sugar. Mix very well.
Spread crumb mixture in the bottom on the spring form, evenly, pressing it over the bottom and half way up the sides.
In a bowl of an electric mixer, beat cream cheese on medium-high speed until smooth and fluffy. Scrape sides and bottom on the bowl and beat for another minute.
Add corn starch and sugar and mix until combined.
Beat in egg until well incorporated.
Add sour cream, vanilla extract, sweetened condensed milk, and heavy cream, beat until all mixed very well.
Add lime zest and mix with a spatula until incorporated throughout.
Transfer batter into the spring form and spread it evenly.
Wash out the bowl from an electric mixer and dry well.
Key Lime Pie Layer:
Beat egg yolks with a whisk attachment, until light and fluffy, about 4-5 minutes.
Start pouring in lime juice into egg yolks while they're mixing. Make sure to pour it in slowly.
Pour in sweetened condensed milk slowly, still beating the egg yolk mixture. After you've added the sweetened condensed milk, beat for a couple more minutes.
Scrape sides and bottom of the bowl and stir gently.
Pour key lime mixture over the cheesecake batter and spread it evenly.
Place in the oven and bake for 20 minutes. After about 15 minutes, start preparing the meringue.
Wash out the bowl of the electric mixer and whisk attachment and dry well.
Pour egg whites into the bowl and add cream of tartar and vanilla extract.
Beat egg whites until soft peaks form and then very slowly add sugar as you continue beating. Beat until stiff peaks are formed, 1-2 minutes.
After the cheesecake baked for 20 minutes, quickly top it off with meringue, sealing the edges as to not let the heat escape. Sealing the edges will help the inside cook better.
Bake for 25 minutes, turn off the oven, open the oven door a little bit and let the cheesecake sit there for 10 minutes.
Take it out of the oven and let it cool to room temperature on a wire rack.
Cover the cheesecake loosely with saran wrap and refrigerate overnight.
Before serving, very gently loosen the meringue from the rim of the springform. (Don't worry, it is very fragile, it will crumble a little bit no matter what you do. Just work slowly and gently to minimize it.) Gently run a greased butter knife between sides of the cheesecakes and the springform to carefully separate it.