Preheat oven to 200°C (180°C fan-forced) and line a 12-hole (1/3 cup capacity) muffin pan with paper cases.
In a large mixing bowl place flours, salt, cinnamon, mixed spice, sugar, walnuts and chocolate together, stirring until combined.
In a large jug, place oil, buttermilk, eggs and vanilla together, whisking until well combined. Pour over the flour mixture and add zucchini, stirring until just combined. Do not overmix, the batter should be thick and lumpy.
Evenly divide mixture between prepared muffin cases and place muffin pan in preheated oven for 20 minutes or until muffins are cooked through. Remove from oven and place on a wire rack to cool.