Preheat oven to 350℉. Line a muffin tin with baking liners (if using) or spray with non-stick baking spray. Set aside.
In a large mixing bowl combine Dutch-process cocoa powder, all-purpose flour, baking powder, baking soda, espresso powder and salt. Stir to combine.
To a bowl, add granulated sugar, canola oil, 2 eggs and 1 egg yolk. Cream together using a wooden spoon, or an electric mixer. Once mixed, add vanilla extract, sour cream (or yogurt), melted unsweetened chocolate and buttermilk. Mix well.
Add the wet ingredients to the dry ingredients and fold to combine. Before the dry ingredients are fully incorporated, add the chocolate chips. Finish mixing to fully incorporate everything. Do not overmix.
Using a #20 disher or a spoon, spoon the batter into the lined or greased muffin pan, filling each cup about two-thirds full to allow room for the muffins to rise as they bake.
Optional: Top each muffin with 1 Tablespoon of mini chocolate chips before baking.
Bake at 350℉ for 15-18min or until a toothpick inserted into the center comes our clean (you're checking for gooey batter vs melted chocolate chips). Cool for 5-10 min in the pan before placing on a wire rack to cool completely.