Brush bottom and sides of springform pan with a little bit of the melted butter and set aside. Adjust oven rack to middle position and heat oven to 350° degrees. Process graham crackers in a food processor until fine; add 1/4 brown sugar and pulse to incorporate. In medium bowl, add processed crumbs with 2/3 cup melted butter and toss with fork until evenly moistened.
Empty crumbs into buttered springform pan and press evenly into pan bottom and about an inch up the sides. Using the bottom measuring cup can help to even out the crust. Bake until fragrant and beginning to brown around edges, 6-8 minutes. Cool on wire rack while making the filling.
For the filling:
In a saucepan dissolve 1 envelop gelatin in 3/4 cup key lime juice over medium heat, about 5 minutes, remove from heat and set aside. Combine 1-1/4 cups of sugar, 4 eggs, 2 egg yolks, 2 tablespoons lime zest, and pinch of salt in a separate bowl and mix well.
Gradually add hot gelatin mixture, whisking to combine. Once all the gelatin and egg mixture is combined pour back into saucepan and cook over medium heat until mixture thickens and is pudding-like, about 7 to 8 minutes. Remove from heat.
With an electric mixer, beat 16 ounces cream cheese until smooth. While mixer is running, slowly add lime mixture and beat until smooth. Transfer to a large, chilled bowl, cover with plastic wrap and refrigerate until completely cooled, stirring every 10 minutes, for about 30-40 minutes.
In an electric mixer, whip the 2 reserved egg whites and remaining 1/4 cup of the sugar on medium high until stiff peaks form. Remove cooled lime pudding from the refrigerator and gently fold egg whites into lime pudding until thoroughly blended.
Pour filling into prepared crust. Cover with plastic wrap and refrigerate until set, about 4 to 24 hours.
When ready to serve run a sharp knife along the side of the pan and remove the spring-form. Slice, cleaning knife between each slice, and serve with fresh whipped cream on top, if desired.
22-24 key limes will produce approximately 3/4 cup of juice, however, it is best to buy extra juice for the assurance you will have enough.
If you can buy key limes individually, buy the 10 you need for the zest and bottled key lime juice for the rest.