In a large bowl, beat butter and sugar until crumbly. Beat in egg. Stir in chocolate. Combine the flour, salt, cream of tartar and baking soda; gradually add to creamed mixture alternately with milk, beating well just until combined.

Pour into a well-greased 7-cup pudding mold; cover. Place mold on a rack in a stockpot; add 1 in. of hot water to pan. Bring to a gentle boil; cover and steam for 2 to 2-1/4 hours or until top springs back when lightly touched, adding water as needed.

Remove mold to a wire rack; cool for 15 minutes. Meanwhile, in a small saucepan, combine the sugar, cornstarch and salt. Stir in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat; stir in the butter, vanilla and nutmeg.

Unmold pudding onto a serving plate; cut into wedges. Serve warm with sauce.