Assemble ingredients and tools. Preheat the oven to 375 degrees. Grease 18 muffin cups.
In a medium bowl or 2-cup liquid measuring cup, whisk together the eggs, oil, buttermilk, coffee, and vanilla extract. Set aside.
Chop the walnuts and set aside.
In a large mixing bowl, sift or whisk together the all-purpose and whole-wheat flours, brown sugar, cocoa powder, baking soda and baking powder.
Stir in the chopped walnuts and chocolate chips.
Add the buttermilk mixture to the dry mixture, and stir quickly to just combine.
Scoop the batter into the prepared muffin cups.
Bake at 375 degrees for about 20 – 25 minutes, or until a toothpick comes out clean when inserted into the muffin. Remove from pan and let cool on a rack. Makes 18 muffins.