Heat the oven to 375°F. Add paper liners to a 12-cup muffin tin, or lightly grease each cup.

Stir the brown sugar and cinnamon together in a small bowl until evenly combined. Set aside.

Add the flour, baking powder, baking soda, and salt to a large bowl and whisk until well blended.

Combine the wet ingredients. In a separate bowl, whisk together the melted and slightly cooled butter, granulated sugar, eggs, vanilla, and milk until smooth.

Add the wet mixture to the dry ingredients and gently fold everything together just until no large streaks of flour remain. Avoid overmixing.

Divide about half of the batter among the prepared muffin cups. Add approximately 1 teaspoon of the cinnamon sugar mixture to the center of each muffin, then lightly swirl it into the batter using a toothpick or skewer.

Spoon the remaining batter over the cinnamon layer, dividing it evenly among all 12 muffins.

Combine the flour, brown sugar, and cinnamon in a small bowl. Add the cold butter cubes and work them into the dry ingredients with a fork until coarse, buttery crumbs form.

Scatter roughly 2 teaspoons of streusel over the top of each muffin.

Place the muffin pan in the oven and bake for 18 to 20 minutes, or until the tops are golden and a toothpick inserted into the muffin portion comes out clean.

Allow the muffins to rest in the pan for a few minutes before transferring them to a cooling rack. They’re especially delicious served slightly warm.