These gooey-good maple bars in a buttery shortbread crust are a favorite on the cookie plate. Be sure to allow plenty of time for them to cool before packing in a tin.
Preheat oven to 350 degrees F and butter an 8-inch square pan. Put the flour, sugar, and salt into a food processor and mix briefly. Add the butter and process about 10 more seconds, until the mixture resembles coarse meal. Add the yolk and water to the dry ingredients with the machine running. Process briefly, just until the dough starts to clump together. (The dough can also be mixed by hand or with an electric mixer.) Empty the dough into the pan and press it into the bottom and halfway up the sides. Prick the dough with a fork in several places. Bake for 25 minutes, then cool on a rack.
Combine the maple syrup, brown sugar, butter, and cream in a small saucepan. Bring to a boil and boil 2 minutes. Stir in the nuts. Scrape the filling into the crust and bake 15 minutes. Cool on a rack.