1. Prepare the Crust:

• Preheat oven to 350°F (175°C).
• In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Stir until evenly moistened.
• Press the crumb mixture firmly and evenly onto the bottom of the 9-inch springform pan.
• Bake for 10 minutes. Let cool completely on a wire rack while you prepare the cheesecake layer.

2. Prepare the Cheesecake Layer:

• Reduce oven temperature to 325°F (160°C).
• In a large bowl, beat the softened cream cheese with an electric mixer on medium speed until completely smooth.
• Gradually beat in the granulated sugar until smooth.
• Add the eggs one at a time, beating lightly on low speed just until each egg is incorporated. Scrape down the sides of the bowl as needed. Do not overmix.
• Stir in the sour cream, 1 tablespoon lime zest, 2 tablespoons lime juice, and vanilla extract until just combined.
• Pour the filling evenly over the cooled crust.

3. Bake the Cheesecake (Water Bath Recommended):

• Wrap the outside of the springform pan tightly with 2-3 layers of heavy-duty aluminum foil. Place the wrapped pan in a larger roasting pan.
• Carefully pour hot water into the roasting pan until it reaches about halfway up the sides of the springform pan.
• Bake for 50-60 minutes, or until the edges are set and the center is almost set (it should still have a slight wobble).
• Turn off the oven and let the cheesecake cool in the oven with the door slightly ajar for 1 hour.
• Remove the cheesecake from the water bath and oven. Remove the foil. Let it cool completely on a wire rack at room temperature.
• Once completely cool, cover loosely with plastic wrap and refrigerate for at least 4 hours, or until well-chilled and firm.

4. Prepare the Lime Mousse Layer:

• In a small bowl, sprinkle the gelatin over the cold water. Let it sit for 5-10 minutes to bloom (it will become jelly-like).
• In a small saucepan, combine the ½ cup lime juice and ¾ cup granulated sugar. Heat over medium heat, stirring, until the sugar is completely dissolved. Do not boil.
• Remove the saucepan from the heat. Add the bloomed gelatin and stir until it has completely dissolved. Let the mixture cool to lukewarm (about 10-15 minutes), stirring occasionally.
• In a medium bowl, beat the 4 oz softened cream cheese until smooth. Gradually beat in the lukewarm lime-gelatin mixture and the 2 teaspoons lime zest until well combined. Stir in the green food coloring, if using.
• In a separate, chilled large bowl, whip the cold heavy cream with an electric mixer on high speed until stiff peaks form.
• Gently fold about one-third of the whipped cream into the lime-cream cheese mixture to lighten it. Then, gently fold in the remaining whipped cream until just combined and no streaks remain. Be careful not to deflate the mousse.

5. Assemble and Chill:

• Pour the lime mousse evenly over the top of the chilled cheesecake layer in the springform pan. Smooth the top with an offset spatula.
• Cover loosely with plastic wrap (try not to let it touch the mousse surface) and refrigerate for at least 4 more hours, or preferably overnight, until the mousse is completely set and firm.

6. Garnish and Serve:

• Carefully run a thin knife around the edge of the cheesecake before releasing the sides of the springform pan.
• Garnish the top with dollops or piped swirls of sweetened whipped cream, thin lime slices or twists, and/or extra lime zest just before serving.
• Slice with a sharp knife dipped in hot water and wiped clean between cuts.