Preheat the oven to 325°F. Line a large, rimmed baking sheet with parchment paper. In a medium bowl, sift or whisk together the flour and cinnamon. Set aside.
In the large bowl of a stand mixer fitted with the paddle attachment, cream together the sugar and butter until completely smooth. Add the egg yolk and vanilla extract; beat to combine. Gradually add the flour mixture, beating until completely smooth and combined. This takes a few minutes. It should look like sticky cookie dough. Scrape down the sides of the bowl periodically while mixing so that the ingredients are well blended.
Drop the dough onto the prepared baking sheet. Using your fingers, press out the dough until it’s about ½-inch thick.
Brush the top of the dough with the egg white. Sprinkle the pecans on top. Cover loosely and chill in the freezer for about 30 minutes or in the fridge for about 60 minutes (Ashley says that it’s not necessary to chill the dough before baking, but I find that it spreads a little bit less if you let it cool before baking).
Remove the cover. Bake for 30-35 minutes, until golden brown and the nuts are toasted.
Let the bars cool completely before slicing into squares. They will continue to firm up as they cool. This recipe yields approximately 22 (2-inch) squares.
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Notes
Store the pecan cookie bars in an airtight container at room temperature for 3-4 days, or wrap tightly and freeze for up to 3 months. Thaw on the counter overnight.Use a stand mixer (not an electric hand mixer) for the best texture. The stand mixer combines the ingredients smoothly, which is really important here. You should not see any little pieces of butter throughout the dough.Room temperature butter is imperative for a smooth, creamy dough.Do not pat the dough too thin -- about ½-inch thick is ideal. Remember, it will spread in the oven, so it will be too thin and crispy if you press it to ¼-inch thickness.