Place the orange in a feed processor or blender, pulse to chop finely.
Sift together flour, baking powder, and salt.
In a large bowl, beat brown sugar, butter and sugar with a electric mixer until light and fluffy. Add egg and chopped orange; beat well. Gradually stir in flour mixture. Add walnuts. Cover and chill for at least 1 hour in refrigerator.
Preheat oven to 375º. Lightly grease a baking sheet.
Drop dough by teaspoonfuls onto prepared baking sheet. Bake for 10-12 minutes or until golden brown. Cool on wire racks.
Orange Glaze: Mix together in a small bowl the confectioners sugar, orange juice, butter and grated orange peel until smooth. Add more orange juice if to thick, more confectioners sugar if to thin.
Spread on cookies and set on waxed paper to set.