Preheat the oven to 350F/ 177C.

In a medium bowl stir together the graham cracker crumbs, sugar & salt. Then pour in the melted butter and mix until everything is evenly moistened and resembles wet sand. Then press the mixture evenly into the bottom and up the sides of a 9-inch pie plate. Press it down using the back of a measuring cup or a drinking glass. Bake for 10 minutes, then transfer to a wire rack to cool.

While the crust cools, add the yogurt to a large bowl and whisk until smooth. Then pour in the sweetened condensed milk, Key lime zest and juice and the salt. Stir well until the mixture is totally combined and creamy.

Pour the filling into the semi-cooled crust. Bake until the center is almost set, but still a little wiggly when jiggled slightly. About 12-15 minutes. Set the pie on the wire rack and cool to room temperature. Then refrigerate until well-chilled, at least 4 hours. (If you are making this ahead, cover the pie directly with plastic wrap sprayed with non-stick cooking spray and refrigerate for up to a day.)

If you are making the whipped cream, add the heavy whipping cream to a chilled mixing bowl and whip on medium-high until stiff peaks form using the whisk attachment. Alternatively, you can whip the cream by hand in a large bowl with a whisk. Pipe the finished cream onto the chilled pie using a pastry bag and tip, a zip-top bag with one end snipped off or dollop it on top with a spoon and spread it out. Garnish with lime slices, if desired, and serve.

Serve cold. Leftovers can be kept covered with plastic wrap and refrigerated for up to 3 days.