Preheat oven to 350°F (175°C). In a medium bowl, combine graham cracker crumbs, sugar, and cinnamon (if using). Pour in melted butter and mix until well combined and the crumbs are moistened.

Press the mixture evenly into the bottom and up the sides of a 9-inch pie plate. Bake for 10-12 minutes, or until lightly golden. Remove from oven and let cool slightly while preparing the filling. Reduce oven temperature to 325°F (160°C).

In a large mixing bowl, whisk together the egg yolks until light yellow and slightly thickened (about 1 minute).

Gradually whisk in the sweetened condensed milk until smooth.

Slowly add the strained Key lime juice and Key lime zest, whisking until the mixture is thick and well combined. The mixture will thicken as the acid reacts with the condensed milk.

Pour the Key lime filling into the cooled graham cracker crust.

Bake for 15-18 minutes, or until the edges are set but the center still has a slight jiggle. Do not overbake.

Remove from oven and let cool completely on a wire rack for at least 1 hour.

While the pie cools, toast the shredded coconut. Spread coconut in a single layer on a dry skillet over medium-low heat or on a baking sheet at 300°F (150°C) for 5-7 minutes, stirring occasionally, until golden brown. Let cool.

In a clean, dry metal or glass bowl, combine egg whites, granulated sugar, cream of tartar, and salt. Place the bowl over a saucepan of simmering water (make sure the bottom of the bowl does not touch the water).

Whisk constantly until the sugar is dissolved and the mixture is warm to the touch (about 3-5 minutes). If using a thermometer, it should reach 160°F (71°C).

Remove the bowl from heat. Using an electric mixer, beat the egg white mixture on high speed until stiff, glossy peaks form (about 7-10 minutes). Beat in the vanilla extract during the last minute.

Once the pie is completely cool, spread the meringue evenly over the top, making swirls with a spatula or the back of a spoon, ensuring it touches the crust all around to prevent shrinking.

Garnish with the toasted shredded coconut. If desired, use a kitchen blowtorch to lightly toast the meringue peaks for a beautiful golden finish.

Refrigerate the pie for at least 4 hours, or preferably overnight, to allow it to set completely.

Slice and serve chilled.