Preheat oven to 350°F.
Beat butter and sugar in large bowl.
Add eggs, maple extract, and 1/3 cup maple syrup, beating until smooth.
In a separate bowl, mix flour, baking powder, and salt.
Add flour mixture to wet ingredients and blend.
Stir walnuts into dough.
Divide dough in half.
Shape each half into 10″ x 3″ rectangle, using floured hands.
Set each loaf 2″ apart on a parchment paper-lined baking sheet.
Bake for 28 minutes or until the dough is set.
Leaving the oven on, remove the biscotti loaves and cool for 15 minutes on cookie sheet.
Using a butcher knife, cut the loaves into diagonal slices, 1/2″ thick.
Place slices on cookie sheet with the cut sides down. Bake for 8 to 9 minutes.
Turn over slices, and bake for 8 to 9 minutes more.
Remove biscotti from oven, and allow to cool completely on a cooling rack. (Save parchment-lined baking sheet.)
In a small bowl, combine powdered sugar, 1/4 cup maple syrup, and milk; stir well to combine.
Using a spoon, coat one side of each biscotti with the glaze. Place on parchment-lined baking sheet.
Repeat with remaining biscotti.
To quicken the setting of the glaze, place tray of glazed biscotti in the refrigerator for a few minutes.