1. Preheat the oven to 425ºF (220ºC). Line a muffin tin with 8 tulip paper liners* and set aside. Chop the chocolate coarsely. Brew the espresso and set it aside.

2. Mix the vegetable oil, brown sugar, vanilla extract, and egg in a large bowl until light and fluffy. Add the sour cream and mix in. Add the espresso and buttermilk and stir to combine. In a second bowl mix the flour, cocoa powder, baking powder, and salt until well combined. Add to the bowl with the wet ingredients and mix until just combined. Add about 2/3 of the chopped chocolate and the frozen berries and fold in. Divide the batter between the paper liners, press some frozen berries into the batter (optional) and sprinkle with the remaining chopped chocolate. Bake the muffins for about 5 minutes, then reduce the heat to 350°F (180°C) and continue baking for about 30-34 minutes or until a toothpick inserted into the center of the muffins comes out clean. Take out of the oven, remove from the muffin tin and let cool down completely on a wire rack.