Preheat the oven to 425℉/220℃ (200℃ fan). Line a standard sized 12 hole muffin tin with muffin liners.
In a large bowl, combine the dry ingredients: granulated sugar, light brown soft sugar, baking powder, baking soda, cinnamon and flour.
Add in the wet ingredients: eggs, oil, butter and milk. Use a hand/balloon whisk to combine everything, taking care not to overmix. It may look quite thick (like pancake batter) but that's what we want!
Scoop the batter into the muffin liners, filling to about ⅔ full. Bake 15-18 minutes, until golden brown and a skewer inserted comes out clean.
Let cool before the next step.
Make the topping: melt the butter in the microwave in a small bowl. In another small bowl, combine the sugar and cinnamon. Take a muffin, brush the top with the melted butter using a pastry brush or similar (you can just spoon it over/dip it in if that's easier for you), before dunking into the cinnamon sugar to cover the top. Repeat for each muffin, then serve!