Preheat oven to 350°.
Combine flour, quinoa, cinnamon, baking soda, baking powder, and salt together in a large bowl.
In a separate bowl, whisk eggs together and then add the syrup, coconut oil and vanilla.
Add the wet ingredients to the dry ingredients and use a large spoon to stir everything together.
Next, chop on 1 1/2 of the plums into small pieces and add the chopped plums to the batter, stirring it so that everything is evenly combined. For the other 1 1/2 plums, thinly slice them and then set the plum slices aside.
Line your muffin pan with muffin liners and then pour batter evenly into the liners. Next, place the plum slices on top of the muffins, adding no more than 2 slices per muffin.
Bake muffins for 20-25 minutes, or until a toothpick comes out clean.
Remove muffins from oven and wait 10 minutes for them to cool a bit before removing them from the tin.