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.