Preheat your oven to 375°F and grease a standard 12-cup muffin tin or line it with paper liners.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt until no large lumps remain.
In a separate smaller bowl (or a large measuring jug), whisk the buttermilk, oil, eggs, and vanilla until fully combined.
Pour the wet ingredients into the dry ingredients and use a spatula to fold them together until the flour just disappears.
Gently stir in 1 1/4 cups of the dark chocolate chips, being careful not to overwork the thick, fudgy batter.
Divide the batter evenly among the 12 muffin cups, filling them about 3/4 of the way to the top.
Sprinkle the reserved 1/4 cup of chocolate chips over the tops of each muffin, pressing them in slightly so they stick.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs attached.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to prevent the bottoms from getting soggy.