Preheat the oven to 350 degrees F.
Grease an 8-inch square baking pan with non-stick cooking spray and line with parchment paper.
In a medium bowl, combine the flour, cinnamon, cocoa powder and salt.
Using an electric mixer, cream the butter and sugar on medium speed until light and fluffy, about 4 minutes.
Beat in the egg yolk and vanilla extract.
Add the dry ingredients to the wet ingredients. Mix on low speed until the dough is just combined.
Spread the dough into an even layer into the prepared baking pan. Lightly whisk the egg white, then brush it over the dough. (You will not need all of it.) Sprinkle the pecans on top, gently pressing them in.
Bake for 30-35 minutes until the edges pull away from the sides of the pan and the cookie that you can see between the nuts is slightly darker than before you baked it.
Let it cool completely before removing from the pan and cutting in 16 squares or your desired size.