Line a jumbo muffin pan with liners and set aside.
In a measuring glass, melt the butter. Then whisk in the buttermilk, eggs and vanilla. Set aside.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda and salt.
Pour the butter mixture into the dry ingredients and fold with a spatula until no lumps of flour remain.
Fold in the chocolate chips.
Cover and refrigerate the batter for 30 minutes. Preheat the oven to 425 degrees.
Scoop the batter into the prepared muffin pan, filling each jumbo muffin cavity ¾ of the way full.
Bake the muffins for 5 minutes, then reduce the temperature of the oven to 350 without opening the door.
Bake for an additional 30 minutes. Remove from the oven and let the muffins cool in the pan for 5 minutes or until they're just cool enough to remove.
Remove from the muffin pan and let cool completely on a wire rack.