Heat oven to 350°F.
Butter bottom of two 8 x 2-inch-round baking pans. Line bottoms with waxed paper or parchment paper rounds. Butter paper.
In a large bowl, stir together the flour, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda and salt until well blended.
In medium-size bowl, whisk together eggs, sour cream and vanilla until blended.
In another large bowl, beat together the melted butter, corn oil and cold water until blended. On low speed, add the flour mixture all at once; then beat in the egg mixture. Continue to beat until the mixture is smooth. Evenly divide the batter between the prepared pans, spreading the batter to edge of pan.
Bake the cakes in 350°F oven for 50 to 55 minutes or until a toothpick inserted in the top centers of the cakes comes out clean. Let the cakes cool in the pans on a wire rack for 15 minutes. Invert cakes onto racks; remove the pans and waxed paper. Let the cakes cool completely.
Place the chopped chocolate in a small microwave-safe bowl. Microwave on 50% power until the chocolate is melted, 2 to 3 minutes; let cool slightly. In medium-size bowl, beat the butter until creamy. Beat in confectioners sugar until light and fluffy. Beat in melted chocolate and vanilla until the mixture is smooth.
With a serrated knife, trim the top of one cake layer to make it level. Place the layer, trimmed side up, on a serving plate. Spread the top with about 1/2 cup of the icing. Place the second cake layer, rounded side up, on top of the first. Using a metal frosting spatula and the remaining icing, spread the icing evenly over the side of the cake. Then spread icing evenly over the top.