Chocolate Sheet Cake - Cook's Illustrated Taste Test

My go to chocolate sheet cake recipe is from a January 2001 Cook's Illustrated.  The newest issue of Cook's Illustrated ran a new recipe for rich chocolate sheet cake, with no mention of the earlier recipe.  I decided to try both to see which I would keep.  The major difference in the cakes was the substitution of butter and buttermilk in the original recipe (cake A) for vegetable oil and whole milk in the new recipe (cake B).  They claim that dairy mutes chocolate flavor, which I've read before.  The frosting was also different.  The new recipe had more chocolate and more butter.  Taste testers preferred the new recipe, but only by a small margin.  I should also note that a few testers preferred the buttermilk brownies over either cake.  

Many thanks to my family and my husband's coworkers for their input!  Thanks to Cook's Illustrated as well for the updated recipe.  

This recipe was more streamlined than the original, another point in favor of the new version.  I made one minor change with the addition of an optional 2 teaspoons of espresso powder.  

"B" Cake:

• 1 1/2 cups (10.5 ounces) granulated sugar
• 1 1/4 cups (6.25 ounces) all-purpose flour
• 1/2 teaspoon baking soda
• 1/2 teaspoon salt
• 1 cup whole milk
• 8 ounces bittersweet chocolate, finely chopped
• 3/4 cup (2.25 ounces) Dutch-processed cocoa powder
• 2 teaspoons espresso powder (optional)
• 2/3 cup vegetable oil
• 4 large eggs
• 1 teaspoon vanilla extract
  

Frosting:

• 1 pound milk chocolate
• 2/3 cup heavy cream
• 2 sticks (16 tablespoons) unsalted butter, softened
  

Directions for the cake:

1.  Preheat the oven to 325 degrees and lightly spray a 13 X 9 inch baking pan with vegetable oil spray.
2.  Whisk sugar, flour, baking soda, and salt together in a medium bowl.
3.  Combine milk, chocolate, cocoa and espresso powder in a large saucepan.  Cook over low heat whisking frequently, until chocolate is melted and mixture is smooth.  Remove from heat and let cool for 5 minutes.

4.  Whisk eggs, oil, and vanilla into chocolate mixture until smooth.

5.  Whisk sugar mixture into chocolate until combined.

6.  Transfer batter to prepared pan and bake for 30-35 minutes until toothpick inserted in the center comes out with a few crumbs attached.  Cool completely.

Directions for frosting:

1.  Combine chocolate and cream in a medium saucepan and heat over very low heat until mixture is uniformly smooth and glossy.
2.  Add butter, whisking once or twice.  Let mixture stand for 5 minutes to melt butter, then whisk until completely smooth.

3.  Refrigerate frosting until cooled and thickened, 30 minutes to one hour.
4.  Whisk frosting until smooth, and spread evenly over the top of the cake.