Combine the flour, baking powder and salt in a small bowl. Place the butter and shortening in a large bowl. Beat with an electric mixer set on medium speed until blended.
Beat in the sugar until light and fluffy. Beat in the lemon zest and vanilla. Reduce speed to low. Beat in the eggs. Beat in half of the flour mixture until combined. Beat in the milk until combined, then the remaining flour mixture
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.
Bake in a preheated 350º oven until a toothpick inserted in the center comes out clean, 50-60 minutes.
Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely. Spread the frosting evenly over the top of the cake.