Spray baking pan(s) with oil; set aside
In a small bowl set over boiling water, melt chocolate. Remove bowl from heat and let chocolate cool for 5 minutes.
In a large mixing bowl, cream butter and sugar until light and fluffy. Add eggs and vanilla; mix well. Add chocolate and combine.
In a medium bowl, combine flour, baking soda, and salt. Add dry ingredients to the first mixture and blend. Add ice water and beat till smooth. Pour batter into the prepared pans.
Bake until a toothpick inserted into the center comes out clean (see Note for baking times). Cool, then frost.
8- or 9-inch squares: 20 to 30 minutes; 13 x 9-inch rectangular cake: 35 to 40 minutes; 2-quart ovenproof bowl: 25 to 30 minutes; 24 cupcakes: 12 to 15 minutes.
In a medium bowl, combine butter, confectioners' sugar, vanilla extract, corn syrup, and milk.
For chocolate frosting, substitute 3/4 cup cocoa for 3/4 cup confectioners' sugar.