Preheat the oven to 350 degrees F. Process the gingersnap cookies in a food processor until you have fine crumbs. If you don't have a food processor, place the cookies in a large sealed bag and bash/roll them with a rolling pin.

Combine the gingersnap crumbs, salt, and melted butter together in a large bowl. The mixture should resemble wet sand.

Press the gingersnap mixture into a 9 inch pie pan, pressing up the sides to form a neat edge. Take the bottom of a glass or similar flat object and press all over the bottom and along the sides. 

Bake the crust for until very fragrant and just set, about 10 mins. Remove to a wire rack to cool.

Combine the egg yolks and lime zest in a medium bowl and beat until pale, light, and fluffy, about 4 mins.

Slowly add the sweetened condensed milk while beating. Continue to beat an additional 4 mins. Mixture should be smooth and thick.

Slowly add the key lime juice while beating. Beat on low speed just until combined. Pour the filling into the crust.

Bake for 8 mins. The filling should be set, but still jiggly. Cool on a wire rack until room temperature and then refrigerate for at least 2 hours.

Garnish with whipped cream, lime slices and lime zest if desired.