Preheat the oven to 200C/400F/Gas 6. Grease a 12-hole muffin pan well, then set aside.
Whisk together the flour, sugar, baking soda, salt, and spices. Set aside.
Whisk together the eggs, oil, yogurt, pumpkin, and vanilla extract. Add the dry ingredients, and mix until just combined. Do not over-mix.
Add half of the batter into a separate bowl, and mix in the cocoa powder.
Alternating between the two batters, spoon into the prepared pan. Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool for 5 - 10 minutes in the pan, then transfer them to a wire rack to cool completely.
Notes
Muffins stay fresh in an air-tight container at room temperature or in the fridge for up to 5 days. They also freeze well for up to 2 months. Thaw overnight in the fridge.