If you want this to come out just like in the photos with the moist and soft cake-like texture, use a scale to measure all ingredients.
Preheat the oven to 350 °F (ca. 180 °C) and put an oven rack in the middle position. Line a 12-cup muffin tin with paper liners. I don't mind if the muffins stick to the papers, but if you do, spray the pan and the liners with non-stick cooking spray.
Mix dry ingredients (flour, salt, and baking powder) in a small bowl.
Add apple cider vinegar to the room-temperature milk.
Use an electric mixer to beat the soft room-temperature butter with cane sugar. The sugar will not melt in the butter. 2 minutes are enough.
Add flax egg (from ground flax, not flakes), olive oil, and vanilla extract. Mix them well.
Add half the dry ingredients and half the “buttermilk” to the batter. Use a spatula or spoon to mix it well. Then add the second half and mix again. Don't need to over-mix it. You'll get a soft, fluffy, pillowy batter.
Gently fold in the chocolate chips.
Use two spoons or an ice cream scooper to arrange the muffin batter evenly. It will be full. I always add a couple of extra chips on top, so they are visible after baking.
Bake for about 30 minutes, until lightly golden. Let the muffins cool in the muffin tin for about 5-10 minutes. They are very soft and moist, especially while hot. Run a knife around the edge of each muffin if necessary before taking them out.