1. Preheat oven to 400ºF
2. Combine the chocolate and the milk in the top of a double boiler and heat over simmering water. Beat until the chocolate has melted and the mixture is smooth.
3. Combine the sugar, salt, and flour and mix well. Add to the chocolate and milk. Beat thoroughly.
4. Pour a small amount of the chocolate mixture into the egg yolks to temper them, and mix well. Add the yolks to the remaining chocolate mixture, and blend thoroughly.
5. Cook over simmering water, stirring constantly, until mixture has thickened, 8 minutes. Add the butter and blend. Remove from the heat, add the vanilla.
6. Spoon the filling into a 9-inch baked pie shell.
1. Beat the egg whites with a tiny pinch of salt until they hold a soft peak. Slowly add the sugar, beating until the whites hold a stiff peak.
2. Cover the warm filling with the meringue,making sure the meringue covers the filling and seals the edge of the crust (this helps prevent shrinking and "weeping").
3. Place the pie on the middle shelf of the oven and bake until the meringue is lightly browned, 10-15 minutes. Cool before serving.