In a medium bowl, mix together graham cracker crumbs, brown sugar, butter, and cinnamon until well combined. Press mixture into bottom and up sides of a 9-inch pie plate.
Bake crust for 10 minutes; let cool completely on a wire rack.
In a large bowl, whisk together condensed milk, eggs, Key lime juice, zest, and salt until combined. Pour into prepared crust.
Bake until filling is smooth and just set, about 20 minutes. (Filling will still be quite jiggly but will set as it cools.) Let cool completely on a wire rack.
In a large bowl, beat cream cheese and granulated sugar with a mixer on medium-low speed until smooth. Reduce speed to medium-low. Slowly beat in one-third of the cream until smooth, stopping frequently to scrape sides of bowl. Gradually add remaining cream; increase speed to medium and beat until stiff peaks form. (Do not overmix.)
Spoon cream cheese mixture into a piping bag fitted with a medium open star tip; pipe over cooled pie as desired. Garnish with lime slices, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/key-lime-pie-2/