Preheat oven to 350 degrees F.
Grease and flour a sheet pan.
To make the cake, cream together sugar, shortening; add buttermilk and vanilla. Add melted chocolate, and eggs, 1 at a time; beat for 2 minutes. Sift together dry ingredients, add to creamed mixture. Beat an additional 2 minutes. Pour into prepared sheet pan. Bake for 35 minutes.
To make the sauce, melt chocolate and butter in saucepan over very low heat. Stir in powdered sugar, alternately with evaporated milk, blending well. Bring to a boil over medium heat, stirring constantly. Cook and stir until mixture becomes thick and creamy, about 8 minutes. Stir in vanilla; serve warm over cake.