Thoroughly grease and flour muffin tins or use paper liners. Set aside.
Combine the flour, cocoa powder, baking powder, sugar and salt in a large bowl.
Combine the milk, melted butter, oil, beaten eggs, and vanilla extract. Stir to combine.
Add the liquid ingredients to the dry ones and mix just until incorporated. The batter will be visibly lumpy. This is okay! It's extremely important not to over-mix the batter, or the resulting muffins can be dense.
With a rubber spatula, gently fold 1 cup of chocolate chips into the batter being careful not to overwork the batter.
Carefully spoon the batter into the prepared muffin pan (I use a small scoop).
Finally, sprinkle the remaining chocolate chips onto the tops of the muffins.
Bake 18-20 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Remove pan from the oven and cool on a wire rack.
I used Hershey's Special Dark - it is SO chocolatey! Use your favorite powdered chocolate.
If you weigh your flour, it should be 260 grams. This will use the least amount of milk listed. If you do not weigh the flour, you may need some additional milk to thin the batter.
You may use your favorite oil. I've used oil as it keeps the muffins moister than using butter alone (since butter is partially water which evaporates - it can leave muffins dry).