Preheat oven to 375 degrees.
Prepare the flax eggs and let sit 10 minutes to thicken.
Add all ingredients (except mix-ins) to the blender or food processor container or to a medium-sized bowl if you are using an emersion blender.
Blend until mixed well and smooth.
Add any mix-ins that you want and stir to incorporate.
Spoon batter into muffin tin, filling to the top.
Bake at 375 degrees for about 18-20 minutes. A toothpick inserted into the center of the muffin should come out mostly clean. It won't be completely dry, but it shouldn't be gooey.
Allow them to cool in the pan on a wire rack for about 5 minutes. Then carefully insert a skinny spatula or knife around the edges and under if possible. Tip the muffins out onto a wire rack to cool upside down for about 5 minutes and then carefully turn right side up to finish cooling.