Preheat oven to 325° F.

To the bowl of a stand mixer fitted with the paddle attachment, add the butter. Beat for 30 seconds until smooth.

Add the sugar and beat until light and fluffy. Add the vanilla extract, egg yolk and the milk. Mix until well incorporated.

With the mixer on a low speed, add the flour a little at a time. Mix only until just incorporated.

Turn the dough out onto an UNBUTTERED 1/2 sheet baking pan (17″ x 12″). Break dough into chunky pieces and scatter over bottom of pan. Press the dough evenly in the pan all the way to the edges. Handle as little as possible. Beat the egg white lightly with a fork, then brush all over the top of the cookie dough. Sprinkle chopped pecans over the dough.

Bake for 30-40 minutes until pale golden brown on top and the dough has set. Do not over-bake. Remove from oven and allow to cool in pan for 1-2 minutes. While cookies are still hot, cut into 48 squares. Remove from pan and place on a cooling rack. When cool, store the cookies in an airtight, covered container. Best if used within 5 days.