In a medium bowl, stir together flour, baking powder, baking soda and salt.
Using an electric mixer, cream butter, 1/4 cup sugar, and brown sugar on medium-high speed for 3 minutes, until light and fluffy. Add egg and beat well.
In a small bowl, whisk together sour cream and maple syrup.
On low speed, add 1/2 dry ingredients to butter-sugar mixture. Pour in 1/3 sour cream mixture. Add remaining dry ingredients in 2 batches, alternating with sour cream mixture. Beat until just blended.
In a small bowl, stir together walnuts, cinnamon and remaining 1 teaspoon sugar.
Pour batter into muffin cups. Sprinkle top of each muffin with walnut mixture. Bake until muffins are golden brown, about 20 minutes. Place tins on wire rack to cool about 5 minutes. Remove from tins and serve warm.
* Dark amber maple syrup is preferred (available online at www.bairdfarm.com).