Preheat and prep: Heat your oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners.
Mix dry ingredients: In a bowl, whisk flour, baking powder, salt, and 1 to 1 1/2 teaspoons cinnamon. Set aside.
Mix wet ingredients: In another bowl, whisk granulated sugar, brown sugar (if using), melted butter (cooled slightly), eggs, milk, and vanilla until smooth.
Combine: Add dry ingredients to wet. Stir gently until just combined.
Don’t overmix—some small lumps are fine.
Fill the liners: Divide the batter evenly among the cupcake liners, about 2/3 full.
Bake: Bake for 16–20 minutes, or until a toothpick comes out clean and the tops spring back lightly.
Make cinnamon sugar: In a shallow bowl, mix 1/2 cup granulated sugar with 1 to 1 1/2 teaspoons cinnamon. Adjust to taste.
Coat the cupcakes: While cupcakes are warm but cool enough to handle, brush tops lightly with melted butter. Gently dip or sprinkle with cinnamon sugar to coat the domes.
Cool completely: Let cupcakes cool on a rack before frosting.
Make the frosting: Beat softened butter until creamy.
Add powdered sugar, 1 teaspoon cinnamon, vanilla, and a splash of milk or cream. Beat until fluffy, adding more milk for a soft, pipeable texture.
Frost and finish: Pipe or spread frosting onto cooled cupcakes. Optionally, sprinkle with extra cinnamon sugar or drizzle with caramel.