Look, key lime is a classic and it can never be improved upon. That being said, sometimes you want a more mellow citrus flavor while staying in the same general family. This grapefruit lime pie hits that note, and the addition of grapefruit marmalade and lime-accented whipped cream really seals the deal. —Matt Lardie - Eat, Write Go
Preheat the oven to 350 F. In a medium bowl stir together the graham cracker crumbs and sugar. Pour in the melted butter and mix with a fork until it resembles moist sand. Pour the crumbs into a 9" pie pan and gently press into the bottom and up the sides until you've formed an even crust. Bake for 15 minutes or until lightly browned, then remove and let cool on a rack while you prepare the filling.
In a large bowl whisk together the sweetened condensed milk, egg yolks, and a pinch of salt. Add the grapefruit and lime juices and whisk until just combined. Pour the filling into the baked crust.
Bake at 350 F for 15 minutes, until mostly set in the center. Remove and let cool to room temperature on a rack, then refrigerate for at least four hours.
To make the topping, add the heavy cream and lime zest to the bowl of a stand mixer fitted with a whisk (or in a large bowl using a handheld electric beater or a handheld whisk). Whip until mostly stiff peaks.
Prep a pastry bag (or large ziploc bag) with a large star tip. Arrange the 8 lime half-moons evenly around the edge of the pie. Pie three stars of the whipped cream in between each half-moon.
Pour the two tablespoons of grapefruit marmalade into the center of the pie, and using the back of a spoon, gently spread into a circle that reaches the edges of the lime half-moons. Serve chilled.