Preheat oven to 180°C/350°F fanbake. Grease 12 muffin pans and line with baking paper or paper cases.
Mix Brown Sugar Topping ingredients in a bowl. Set aside.
To make the muffins, melt butter in a medium pot or in the microwave, then remove from heat and whisk in eggs, sugar and vanilla. Combine milk and baking soda and add to egg mix with all remaining ingredients. Stir to just combine (don’t beat or the muffins will be tough and rise unevenly).
Divide mixture between muffin pans then sprinkle with Brown Sugar Topping. Bake until the muffins are risen, lightly golden and bounce back when pressed (20 minutes). Allow to stand for 10 minutes before turning out.