Cream the margarine + sugar before adding eggs. Then after the eggs add the yogurt and roughly mix it. Don't over-mix.
You have 10 seconds. Do not over-mix. You want it just combined.
Fresh is best, frozen is fine.
If your berries are frozen and you use silicon muffin cups, then this will take about 35 mins to cook. Either way, do the toothpick or chop stick test to see if they're done.
If you're using frozen berries and want to reduce the purple marbling, you can try dusting them with flour or any of the stupid tips online, but if you're really that concerned about it then just get fresh berries which mix in a lot cleaner.
Last updated 2021-03-31