Grind graham crackers until fine. Mix in sugar. Pour into a bowl and mix in melted butter. Put mixture into a 9 inch/23cm pie pan set on a baking sheet. Using a 1/4 cup measuring cup to press crumbs up the sides and all around pan. Bake in heated oven for 8 minutes. Remove from oven and cool completely on a rack.
Heat oven to 350 F/180 C. When oven comes to temperature, make filling. Place egg yolks in a bowl and whisk for a minute until smooth. Add condensed milk and mix. Add key lime juice and mix. Pour immediately into cooled crust. Smooth over top and spread evenly with a spatula. Bake in heated oven for 15 minutes. Remove from oven and cool completely on a rack. Cover and refrigerate for at least a few hours before serving.
Optional - can be topped with meringue or whipped cream.