• Preheat oven to 350°F. Grease a 9x5-inch loaf pan; line with a parchment paper sling. (see notes)
• In a large bowl, stir together flour, granulated sugar, baking powder, and salt. Make a well in the center of the flour mixture. 

• In a medium bowl combine milk, butter, eggs, and vanilla. Add milk mixture all at once to flour mixture. Stir just until moistened (batter should be lumpy).
• In a small bowl combine cocoa powder and espresso. Stir 11/2 cups of the batter into the cocoa mixture until combined.
• Using a separate large spoon for each batter, alternately drop spoonfuls of plain and chocolate batters into prepared pan. Using a butter knife or thin metal spatula, gently swirl batters to marble (do not overmix).
• Bake until a toothpick comes out clean, about 70 minutes. Using a wooden skewer, poke warm loaf across the top at 1-inch intervals.
• Brush loaf with Espresso Simple Syrup. (See the instructions below.)
• Cool the loaf in a pan on a wire rack for 5 minutes. Use parchment to lift the loaf from the pan; cool completely on a wire rack.
• Frost with Mascarpone Frosting. If you like, dust with additional cocoa powder.

Espresso Simple Syrup

• In a small bowl whisk together hot brewed espresso and sugar until sugar is dissolved. If you like, stir in coffee-flavor liqueur.

Mascarpone Frosting

• In a large bowl, combine mascarpone, powdered sugar, coffee-flavor liqueur (if using), vanilla, and salt. Beat with a mixer on medium-high speed until combined.
• Slowly add heavy cream while continuing to beat until mixture thickens to a spreadable consistency.