Preheat oven to 180 °C.
Line a muffin tin with paper baskets.
In a mixing bowl combine flour, cocoa powder, baking soda and stevia.
In another medium bowl whisk the egg, then mix with the yogurt, coconut oil and vanilla extract, until smooth.
Add the wet mixture to the dry one and gently stir with a wooden spoon until just combined. Fold in the chocolate chips if used.
Divide the batter equally between the muffin pan baskets.
Bake for 20-25 minutes until an inserted skewer comes out clean.
Remove the tin from the oven and let the muffins cool in the pan for 5 minutes.
Transfer the muffins from the pan to a wire rack to cool completely.