Preheat the oven to 180C/356F. Line 9-10 X 80ml muffin holes with muffin liners.
Add the buckwheat flour, baking powder, baking soda and salt to a medium bowl and whisk to combine.
Add the cocoa to large heatproof bowl and pour over the boiling water. Whisk to combine and form a thick paste then add the light brown sugar, oil and yoghurt. Whisk until the mixture is smooth and cohesive.
Add the flour to the wet ingredients and whisk until a batter forms. It should be mid chocolate in colour (darker if you use dark buckwheat flour) and the consistency of a thick cake batter or pancake batter. If you lift some batter up with a spoon it should fall back down in thick ribbons, but it should be thick enough to linger on the spoon. If your batter is thick, hard to stir and very slow moving, add the milk. If you are using light buckwheat flour, you are more likely to need the milk than if you are using dark buckwheat flour (which absorbs less).
When you are happy with the consistency, add 100g of the chocolate chips. Reserve 25g chocolate chips for topping the muffins (see notes).
Spoon the batter equally into the muffin liners, coming up to just under the rim of the liner. Top with the remaining chocolate. Bake for 15-20 minutes or until cooked through.
Allow the muffins to cool completely before transferring them to an airtight container. In hot weather I recommend storing them in the fridge.