Grease and dust with flour a 9"x5"x3" loaf pan. Set aside.
In a medium mixing bowl, whisk together the dry ingredients ( flour, baking powder, salt and spices ). Set aside.
In a mixing bowl of a stand mixer, beat together the wet ingredients ( eggs, sugar, butter, milk, pumpkin and vanilla ). Beat until smooth, about 1 to 2 minutes.
Slowly add the dry ingredients and beat on low until just combined.
Melt chocolate in a double boiler.
Scoop out ⅓ of the pumpkin batter into a small bowl and add the melted chocolate. Stir with a wooden spoon until uniform in color.
To achieve the marbled effect, scoop ¼ cup pf pumpkin batter onto the bottom of your loaf pan. Top with 2 tablespoons of chocolate batter and do not stir. Repeat until you used up both of the batters.
Bake for 65 to 70 minutes, or until the toothpick inserted in the center comes out clean.
Cool cake in pan for 10 minutes, then transfer onto a cooling rack.
Cool completely before glazing.
Make the glaze, by melting the butter in a double boiler. When the butter has only few pieces left to melt, add cocoa powder and whisk until all combined. Remove from heat.
Add milk and vanilla and whisk well.
Add powdered sugar, ¼ cup at a time and whisk well.
Pour over cake.
Notes
Recipe for loaf cake adapted from Better Homes and Gardens Magazine November 2011 issue Recipe for cocoa glaze: Crunchy Creamy Sweet original