Your batter should be a soft dropping consistency so you can put it into your muffin cases easily. If it seems too thick, just add a tiny bit more milk.
The baking time can vary a bit depending on what brand of yogurt you use and/or how big your muffin cases are.
Just check every couple of minutes after 17 minutes until your muffins are nicely browned and a skewer pushed into the centre of a muffin comes out fairly clean (there might be a bit of chocolate on the skewer but there shouldn’t be any raw batter).
I’ve found cooking time to be as little as 17 minutes and as long as 25 minutes depending on the size of my muffins.
These muffins will keep quite well for a few days in an airtight container, but they also freeze well. Simply freeze in Ziploc bags or a plastic/glass container with a lid. Defrost and then reheat for 15 seconds in the microwave or a few minutes in a warm oven.
You can easily make blueberry muffins instead with this same recipe! Simply add a tiny bit more sugar to the batter (if you like - say 1/2 cup instead of 1/3 cup) and replace the choc chips with 1.5 cups of blueberries.
Or make half chocolate, half blueberry muffins! Both are delicious and always turn out well.