Key Lime Cheesecake

Lime custard:

6 large egg yolks  
3/4 cup sugar  
6 tablespoons fresh Key lime juice or regular lime juice  
1 teaspoon grated Key lime peel or regular lime peel

Whisk all ingredients in heavy small saucepan over medium heat until custard thickens and boils for 30 seconds, about 8 minutes. Cool to room temperature, stirring occasionally (mixture will thicken).

Crust:

1 3/4 cups graham cracker crumbs (about 12 whole graham crackers)  
1/4 cup sugar  
1/2 teaspoon salt  
1/2 cup (1 stick) unsalted butter, melted
 
Preheat oven to 350°F. Wrap 3 layers of foil around outside of 8- to 8 1/2-inch-diameter spring form pan with 3-inch-high sides. Butter the pan. 
Blend first 3 ingredients in medium bowl. Mix in butter until moistened. Press crumb mixture evenly onto bottom and 1 1/2 inches up sides of prepared pan. Bake just until set, about 5 minutes. Cool completely.
 
Filling:
 
2 (8-ounce) packages cream cheese, room temperature  
2/3 cup plus 3 tablespoons sugar
2 large eggs  
3 tablespoons fresh Key lime juice or regular lime juice  
1 tablespoon grated Key lime peel or regular lime peel
 
Place cream cheese, 2/3 cup sugar, eggs, lime juice, and lime peel in processor; blend well.
Spoon custard into crust; smooth top. Set cheesecake in large baking pan. Add enough hot water to baking pan to come 1 inch up sides of cheesecake pan. Bake until almost set but not puffed and center moves slightly when pan is gently shaken, about 45 minutes.

Topping:

1 16-ounce container sour cream  
3 tablespoons sugar

Meanwhile, stir sour cream and remaining 3 tablespoons sugar in medium bowl to blend.
Carefully spoon sour cream mixture over hot cheesecake; smooth top. Bake until topping sets, about 10 minutes. Cool 10 minutes. Run knife around sides of pan to loosen. 

Cool cheesecake completely. Cover and refrigerate overnight. Can be made 2 days ahead. Keep refrigerated. Release pan sides from cheesecake; transfer to platter. Garnish with lime slices and serve.