Preheat oven to 350 degrees.
Combine milk and vinegar; set aside.
Melt chocolate in saucepan, stirring over low heat.
Sauté onion in 2 T oil in skillet until very soft and buttery sweet.
Using electric mixer in large bowl, beat remaining oil with sugar, eggs, and vanilla until thoroughly mixed and fluffy, 2 - 3 minutes.
Beat in warm melted chocolate and sautéed onions.
Mix flour with baking soda and salt; stir into batter alternately with milk.
Divide batter evenly into two well greased and floured 8 inch round cake pans.
Bake 40 minutes or until toothpick inserted into center comes out clean.
Cool 15 minutes then invert onto wire racks to cool.
Frost and fill with fudge icing.

Fudge Icing:
Melt chocolate with butter in saucepan, stirring often over very low heat.
Mix in hot water then turn into mixing bowl.
Beat in powdered sugar.
Spread icing on cake while still warm.