Preheat the oven to 180°C (350°F). Place a rack in the center of the oven. Line a 12-count muffin pan with paper liners.

If the coconut oil is solid, melt it and let it cool slightly while you prepare the other ingredients.

In a large bowl, combine 1 3/4 cups oat flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon salt, and 3/4 cup sugar (or sugar substitute). Stir until evenly mixed.

In a separate bowl, whisk together 1/2 cup melted coconut oil, 1 cup Greek yogurt, 2 large eggs, and 1 teaspoon vanilla extract until smooth.

Pour the wet mixture into the dry mixture and stir gently with a spatula or spoon just until the ingredients are combined and no large streaks of flour remain. Do not overmix.

If using, fold 1/2 cup chopped chocolate into the batter until evenly distributed.

Divide the batter evenly among the 12 lined muffin cups, filling each about three-quarters full.

Bake for 20–23 minutes, or until a skewer inserted into the center of a muffin comes out mostly clean.

Remove the pan from the oven and let the muffins cool in the pan for 10 minutes. Then transfer the muffins to a wire rack and let them cool completely.