1. Preheat oven to 350 degrees F.
2. Spread the walnuts on a cookie sheet and bake for 4 minutes then toss the nuts around and bake for another 3 or 4 minutes. Let the nuts cool completely before grinding them up in a food processor along with the 1/4 cup sugar. Grind the nuts fine but not too fine. They should not be powdery. Remove and set aside.
3. To the food processor add the citing sugar, baking powder and flour.
4. Pulse the dry ingredients together.
5. Pulse in the butter until well combined.
6. Finally add the nut mixture and pulse that in several times to combine thoroughly.
7. Add the egg yolk, vanilla extract and water that have been whisked together. Mix together until a soft dough forms.
8. Wrap dough and chill for at least one hour. Roll out to a little less than a 1/4 of an inch thick. Cut out with a 2 1/2 inch cookie cutter.
9. Bake at 350 degrees f for 11-12 minutes or until the edges just start to brown. Cool completely on a wire rack.

To prepare the Maple Frosting

1. In a very small saucepan, simmer the maple syrup.
2. Simmer fro only a few minutes until the volume has reduced by half. Watch this carefully, it can easily burn if unattended. Cool to room temperature.
3. Slowly beat together the icing sugar butter and vanilla extract.
4. Add the maple syrup and beat until smooth. If the frosting doesn't seem thick enough add another few tbsp of icing sugar.
5. Sandwich the cookies together with the frosting.