Melt chocolate in the microwave in a microwave safe bowl. Once melted, allow the chocolate to cool 1-2 minutes.
Sift cocoa powder and espresso powder (if using) into the 1 cup of batter, and mix until combined.
Add in milk, and mix until combined.
Lastly, mix in melted chocolate, mixing until combined.
Spoon 1/3 of the vanilla batter into the bottom of the prepared baking pan.
Dollop half of the chocolate batter on top.
Spoon 1/3 of the vanilla batter on top.
Dollop the remaining chocolate batter on top.
Spoon the remaining 1/3 of vanilla batter on top of the chocolate.
With a knife, swirl the batter together.
Bake at 350 for 15 minutes, then reduce the heat to 300 and bake for another 55- 60 minutes, or until a cake tester comes clean. If the top of the pound cake is browning too quickly, tent a piece of non-stick foil on top.
Allow to cool 10 minutes before removing from the pan.