In a food processor or blender, grind the almonds into a coarse powder (don't process them too much or you'll have almond butter! It's ok if it's not a super-fine powder). Transfer to a mixing bowl then grind the oats into a fine powder and add them to the bowl as well.
Add the buckwheat flour, arrowroot powder, sugar, baking powder, baking soda, cinnamon and salt. Mix very well to combine.
Preheat the oven to 190 C and line 12 muffin moulds with cupcake liners.
In a bowl, mash the bananas very well, then add the oil, vinegar, milk and vanilla. Stir till smooth and well combined. Pour into the dry ingredients.
Fold gently to combine making sure there are no bits of dry flour at the bottom, but do not over-mix. Fold in the mix-ins, if using.
The batter is sticky and thickens quite quickly once mixed. Divide it equally into the muffin moulds. Bake for 20 minutes, then turn the oven down to 175 C and bake for another 5 to 10 minutes, till the tops are browned and a toothpick poked in the center of the muffins comes out clean.
Allow to cool completely, before storing in an airtight tin at room temperature for 2 to 3 days, then in the fridge for up to a week. Happy baking!