Preheat oven to 325 degrees.
Grease a 9 inch pie dish or springform pan and place on a baking sheet.
Add graham crackers to a food processor and pulse until it is small crumbs.
Pour into a bowl and add melted butter.
Mix well then press into the pan. Press onto the bottom and up the sides, using a drinking glass to press it down well.
Bake for 8 minutes then remove and let cool completely.
Adjust oven temperature to 350 degrees.
Heat a double boiler on medium heat over simmering water.
Melt butter and dark chocolate together, stirring often, until smooth.
Remove from heat and let cool slightly. Pour into a large mixing bowl.
Whisk in vanilla, salt, and 1/2 cup sugar then add all three egg yolks.
Sift the flour and cocoa powder evenly into the chocolate mixture then stir to combine.
In a large bowl, beat the remaining three egg whites with a mixer on medium until foamy, adding in the 1/2 cup sugar.
Increase the speed to high and beat until stiff peaks form, about 3 minutes.
Fold the egg whites gently into the chocolate mixture.
Spread into the pan, smoothing the top.
Bake for 30-35 minutes until the top is cooked but the center can still be gooey.
Let cool completely, about an hour, until you can remove from pan and place on a platter.