Prep the pan and oven. Heat your oven to 375°F (190°C). Line a 12-cup muffin pan with paper liners or lightly grease it.
Mix the dry ingredients. In a large bowl, whisk spelt flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and 1 tsp cinnamon. Break up any sugar lumps with your fingers if needed.
Combine the wet ingredients. In a separate bowl or large measuring jug, whisk eggs, yogurt, milk, oil, and vanilla until smooth.
Make the cinnamon swirl. In a small bowl, mix brown sugar and cinnamon.
Stir in melted butter if using. It should look like damp sand.
Bring the batter together. Pour the wet mixture into the dry ingredients. Stir gently with a spatula until just combined. Stop when you no longer see dry flour. A few small lumps are fine.
Layer and swirl. Add about 1 heaped tablespoon of batter to each muffin cup.
Sprinkle a scant teaspoon of the cinnamon swirl mixture over the batter. Add another tablespoon of batter on top, then another sprinkle of cinnamon sugar. Use a toothpick or butter knife to gently swirl, making a figure eight.
Top and bake. If you like, sprinkle the tops with coarse sugar.
Bake for 16–20 minutes, or until the muffins are risen and a toothpick inserted into the center comes out with a few moist crumbs, not wet batter.
Cool. Let muffins rest in the pan for 5 minutes, then transfer to a wire rack to cool completely. Spelt muffins are more tender when cooled, but they’re wonderful slightly warm.