Make crust: Preheat oven to 350°F. Grind crackers and sugar in a food processor. Add butter; pulse until moist. Transfer to a 9-inch pie plate; press into an even layer. Bake until firm, about 10 minutes.
Make filling: Melt butter with chocolate in a bowl set over a pan of simmering water. Remove from heat; whisk in flour, sugar, vanilla and salt. Whisk in eggs and white, 1 at a time, until smooth. Pour into crust. Bake until top is puffed and filling is set in center, 35 to 40 minutes. Let cool. Serve at room temperature, or cover and chill to serve cold.