Preheat your oven to 180C/350F/Gas mark 4 and line or lightly grease a 12 hole muffin tin.
Sift or mix the flour, cocoa powder, baking powder, baking soda, salt and sweetener into a medium sized mixing bowl and place to one side.
In a large jug or small mixing bowl mix together the oil, vanilla, buttermilk and the eggs until they are well combined.
Pour the wet ingredients into the bowl of dry ingredients and very gently fold them together.
Gently fold in the chocolate chips.
Spoon the mixture equally into the 12 muffin holes and pop them into the oven and bake for 15 minutes.
Check that they are baked through by inserting a pick. If it comes out clean they are baked through if it doesn't give them another couple of minutes.
Once baked through place them on a cooling rack to fully cool down.
Kept in an airtight tin at room temperature these muffins will be good for 5 days. They are also freezable and will be ok in the freezer for up to 3 months. Defrost fully before consuming.