Preheat the oven to 350°F. Line 12 muffin cups with paper liners, and coat the liners with cooking spray.
In a medium bowl, whisk together the cocoa powder, flour, baking powder, and salt. In a separate bowl, stir together the butter or coconut oil, eggs, and vanilla. Mix in the Greek yoghurt, stirring until no large lumps remain. Stir in the pure maple syrup. Alternate between adding the flour mixture and the milk, beginning and ending with the flour mixture, and stirring just until incorporated. Combine the flour mixture in three parts.
Divide the batter between the paper liners. Bake at 350°F for 20 minutes or until the centres feel firm to the touch and a toothpick inserted into the centre comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
For the frosting, add the cocoa to a small bowl, and make a well in the centre. Pour in the milk, vanilla, and vanilla creme stevia. Stir until thoroughly combined. Spread on top of the cooled cakes.