Preheat your oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, brown sugar, and melted butter. Stir until well combined and resembles wet sand. Press mixture firmly into the bottom and up the sides of a springform pan. Bake for 8-10 minutes until set and golden, then cool.

In a large bowl, whisk together egg yolks and key lime juice until well combined. Gradually add in sweetened condensed milk, mixing until smooth and creamy. Pour this mixture over the cooled crust. Bake at 350°F (175°C) for about 15-20 minutes or until set.

After baking, allow the pie to cool at room temperature for about 30 minutes. Then, refrigerate for at least 6-8 hours or overnight.

In a chilled mixing bowl, pour in the heavy whipping cream and add powdered sugar. Beat with an electric mixer until stiff peaks form, usually 3-5 minutes.

Spread the whipped cream over the chilled pie, smoothing it out with a spatula. Garnish with lime zest or fresh lime slices if desired. Slice into portions and enjoy.