Place a rack in the middle of the oven and preheat it to 180 °C / 350 °F. Line a muffin tray with paper muffin cases.
Mix buttermilk, oil, maple syrup, egg, allspice, and cinnamon powder in a large bowl. Stir in the grated carrots, chopped walnuts, and your vanilla muffin mix (or your favorite vanilla muffin dry ingredients). Stir in half of the chocolate chips.
Fill muffin cases with the batter. Sprinkle the rest of the chocolate chips on top. Place in the preheated oven on the middle rack and bake for 25 - 30 minutes at 180 °C / 350 °F. Transfer the baked muffins to a wire rack to cool to room temperature, then serve.