In a heavy iron skillet, over medium heat, stir 1 cup sugar until melted and golden brown. Reduce heat to low and mix in boiling water. Set aside.
In a heavy saucepan, melt butter. Remove from heat. Add flour and salt into butter mixing thoroughly. Stir in 1/4 milk. Return pan to stove, on low heat, and add remaining 1/4 cup milk and caramel syrup. Cook, stirring constantly, until thickened. Put egg yolks in a small bowl. Add about 1/2 of the hot mixture to egg yolks. Pour the yolk mixture back into remaining caramel mixture. Cook 1 minute and pour into baked pie shell.
Beat egg whites with cream of tartar until foamy. Add remaining 1/2 cup sugar, very slowly, beating until egg whites are stiff. Spread over filling and bake at 400 degrees for 10 minutes, or until meringue is lightly browned.