In a small bowl, stir together flour, baking powder, and baking soda and set aside.
In a medium bowl, beat the butter until smooth, then slowly add maple syrup, beating constantly. Beat in sour cream and egg. Stir in walnuts. Add the combined dry ingredients and stir just until blended.
Spoon into the prepared muffin tins, filling two-thirds full. Bake 12 to 16 minutes. Cool in tins 5 minutes, then move to a cooling rack. Dip muffin in syrup and dust with confectioners' sugar. Serve warm.