Heat oven to 350°F and line a muffin tin with liners or lightly grease.
In a bowl, whisk the Greek yogurt, eggs, melted butter or oil, and vanilla until smooth.
In a separate bowl, combine the flour, cinnamon, brown sugar, monk fruit,baking powder, and salt.
Add the dry ingredients into the wet mixture and stir until just combined. Do not overmix. Fold in any add-ins if using.
Spoon batter into muffin cups, filling about ¾ full. Bake for 16–20 minutes, or until a toothpick inserted comes out clean.
Let rest in the pan for 5 minutes, then transfer to a wire rack to cool completely.