Place the graham crackers, brown sugar, and salt in a food processor. Process until the mixture forms fine, even crumbs.

Transfer the crumb mixture to a large mixing bowl and add the melted butter. Stir until the crumbs are evenly moistened and hold together when pressed.

Press the crumb mixture firmly into the bottom of a 9-inch springform pan. Use the bottom of a measuring cup or your fingers to create an even layer. Press the mixture up the sides of the pan about 2 inches high to create a sturdy border for the filling.

Place the prepared crust in the freezer for 1 hour to set completely.

Remove the cream cheese from the refrigerator and let it sit at room temperature for 1 hour. The cream cheese should be soft but still slightly cool to the touch.

Zest 2 key limes directly into the measured granulated sugar. Use your fingertips to rub the zest into the sugar until fragrant and the sugar takes on a slightly green tint, about 1 minute. This releases the aromatic oils from the zest into the sugar.

Using an electric hand mixer, beat the cream cheese, mascarpone, and key lime sugar on medium speed until the mixture is creamy and well combined, about 2 minutes. Scrape down the sides of the bowl as needed.

Add the key lime juice and vanilla extract. Beat on medium speed until fully incorporated, about 30 seconds. Scrape down the bowl.

Add the heavy cream and mix on medium-low speed until the cream is well incorporated and the filling looks soft and creamy. The beaters should leave visible tracks in the mixture, about 90 seconds. Do not overbeat.

Remove the prepared crust from the freezer. Pour the cheesecake filling into the graham cracker crust. Use an offset spatula to smooth the top, creating an even surface.

Cover the cheesecake tightly with plastic wrap, making sure the plastic does not touch the surface of the filling. Refrigerate overnight or for at least 8 hours until completely set.

Place the heavy cream and powdered sugar in a mixing bowl. Beat with an electric mixer on medium speed until soft peaks form. Do not overwhip.

Transfer the whipped cream to a piping bag fitted with a star tip. Pipe decorative swirls around the top of the chilled cheesecake.

Place one key lime half-slice on top of each whipped cream swirl. Serve immediately or refrigerate until ready to serve.

To serve, run a thin knife around the edges of the springform pan to loosen the cheesecake. Remove the sides of the pan. Use a sharp knife dipped in warm water and wiped clean between cuts to slice neat portions.

The cheesecake can be stored covered in the refrigerator for up to 5 days. Once decorated with whipped cream and key lime slices, it is best served within 1–2 days for optimal appearance and texture.