Brown butter in a pan on stove top over medium heat, stirring constantly. Remove from heat as soon as desired level of color is reached. Let cool for 5-10 minutes.
Mix the brown butter, sugars and vanilla. Stir in the egg.
Mix in the flour and cornstarch until well blended. Stir in pecans.
Chill dough for 30 minutes minimum.
Make cookie dough balls about 1½ Tablespoons and place on a parchment paper lined baking sheet.
Mash tops of dough balls lightly with a flat surface.
If desired a pecan half can be pressed into the center of the cookie.
Bake at 375 degrees for 8-10 minutes. Remove from oven when lightly browned on edges and centers still appear moist.
Allow to cool on pan about 2 minutes and then move to a wire rack to complete cooling.
Notes
Pecans can be toasted over medium heat on stove top or in oven about 8-10 minutes, or until fragrant. Stir several times to promote even toasting.
Egg can be brought to room temperature by sitting in a bowl of warm water.