Transfer one-third of the batter to a small bowl. Whisk in the cocoa until blended. Pour half of the vanilla batter into a greased and floured 9x5-inch loaf pan. Drop half of the chocolate batter by tablespoonfuls onto the vanilla batter. Pour the remaining vanilla batter over the chocolate batter. Drop the remaining chocolate batter over the vanilla batter. (For best marbling effect, drop the remaining chocolate batter in different locations than on the previous layer.) Swirl the batters with a butter knife to create a marbled effect.