Preheat oven to 350º F. Prepare rimmed kitchen sheet pan with nonstick baking spray or by coating with shortening and dusting lightly with flour, being sure to remove all excess flour.
Prepare The Best Chocolate Cake batter as directed and pour into rimmed sheet pan. Use an offset spatula to make sure the batter is distributed evenly. Place in the oven and bake for 12-15 minutes or until the cake slightly pulls away from the edges and a toothpick comes out clean when inserted into the center of the cake. Allow the cake to cool slightly, about 10-15 minutes.
While the cake is cooling, prepare the Fudge Icing recipe as directed. Pour the icing over the top of the cake, making sure to pour over the entire cake and allow it to coat. Only spread with a knife or offset spatula if absolutely needed because it will change the look of the icing on the cake.
Allow the cake and icing to cool completely, about 30 minutes.