Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it lightly with cooking spray.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. This should take about 2-3 minutes.
Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, alternating with the milk. Stir until just combined; be careful not to overmix.
In another small bowl, mix the ground cinnamon, brown sugar, and chopped walnuts (if using) to create the cinnamon swirl mixture.
Fill each muffin cup halfway with the batter, then add a spoonful of the cinnamon swirl mixture on top. Use a toothpick or knife to swirl the cinnamon mixture gently into the batter.
Top each muffin with the remaining batter until the cups are about 3/4 full.
Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
Optionally, dust the cooled muffins with powdered sugar before serving.
Notes
Optional to dust with powdered sugar before serving.