Combine vanilla wafers and ¼ cup dark brown sugar in a food processor and pulse until crumbled.
Pour in the melted butter into the crumbled graham crackers and pulse until combined.
Spread the graham cracker mixture into the bottom of a 9 inch springform pan and press into a crust across the bottom and halfway up the side.
Cover the springform pan with plastic wrap and place in the freezer while you make the cheesecake filling.
Using a stand or hand mixer and a large mixing bowl, beat together the cream cheese, remaining ¾ cup dark brown sugar and key lime zest on medium to high speed until smooth and creamy, about 5-7 minutes. Be sure to beat until all ingredients are smooth and creamy, you don’t want the sugar to be gritty in the mixture.
Beat in the sour cream and key lime juice on low to medium speed until combined, about 2-3 minutes.
Once combined and smooth, gently fold in 4 cups of the whipped topping.
Remove the crust from the freezer and take off the plastic wrap.
Scoop the cheesecake filling into the crust and gently spread it evenly on the crust then recover well with plastic wrap.
Refrigerate for 6 hours or overnight.
When ready to serve, remove the ring from the pan, pipe the remaining whipped topping in dollops around the edge and garnish with fresh lime zest and slices, optional.