Prep pan: Generously line a 9x5-inch loaf pan with plastic wrap. Make sure there is enough hanging over the sides to wrap across the top.

Prep filling: In a tall mixing bowl, beat the cream cheese and powdered sugar with an electric mixer until smooth. With the mixer on low speed, slowly add 1 cup of the heavy cream. Do not add it all at once or the filling will become lumpy. With the mixer on medium speed, whip the mixture until stiff peaks form.

Finish filling: Save a pinch of lime zest for topping the cake. Add the remaining zest, butter, and the salt to your cream cheese mixture. With the mixer on low speed, slowly pour in the lime juice. Once the mixture is fully combined, set aside.

Form cake: Spread about 1 cup of filling in the bottom of the loaf pan. Use a spatula to spread it evenly. Top the filling with a layer of graham crackers, breaking them if necessary to make them fit (save broken pieces for topping later). Repeat the filling and the graham cracker layers 3 more times, until you have 4 layers of filling and 4 layers of graham crackers (ending with graham crackers). Cover with the plastic wrap and freeze for 2-3 hours.

Prep frosting: Add the remaining 1 cup of heavy cream to a tall bowl and beat with an electric mixer on medium until medium peaks form. Add the marshmallow creme and beat until combined. Don't worry if the cream looks a little lumpy -- the marshmallow will melt into the whipped cream as the cake sets.

Frost and garnish: Remove the cake from the freezer, uncover the plastic wrap, and invert the pan onto a plate. Remove the plastic wrap from the cake. Frost the cake with marshmallow whipped cream and sprinkle the top with the reserved graham cracker bits and lime zest. Refrigerate until ready to serve.