Preheat oven to 375 degrees.
In heavy saucepan on low heat, melt broken chocolate with butter, stirring constantly until smooth; set aside.
Beat egg yolks for 1 to 2 minutes, gradually adding 1/2 c sugar and vanilla.
Beat in chocolate mixture.
Fold in flour and nuts.
Beat egg whites with cream of tartar until foamy.
Gradually add remaining 1/4 c sugar, beating until stiff peaks form.
Blend a fourth of the egg whites into the chocolate mixture; fold in remaining egg whites.
Pour into 9 inch springform pan lined with parchment paper.
Bake 30 - 35 minutes or until sides start to separate from pan but center is still moist.
Cool on rack.
Remove torte from pan.
Frost with Chocolate Rum Glaze.

Chocolate Rum Glaze:
In heavy saucepan on low heat, melt broken chocolate with butter, stirring constantly until smooth.
Remove from heat.
Stir in corn syrup and rum.
Place torte upside down on a rack over tray to catch excess glaze.
Spread a very thin layer of glaze over top and sides of torte to set surface.
Chill 15 minutes to firm glaze.
Reheat remaining glaze to thin and pour over top and sides of torte.
When glaze is firm, remove cake from rack to large plate.
Decorate top and sides with shaved chocolate or ground walnuts.
For shiny glaze, store cake at room temp until serving time.