To Make Crust:

Preheat oven to 350 degree F.  Spray a 9-inch pie pan with nonstick cooking spray.  Set aside.  In a small saucepan, melt butter over medium heat stirring often for 5-8 minutes.  Watch carefully and stir often as butter begins to show brown flecks on the bottom of the pan.  Butter will begin to pop and develop a nutty smell. Butter is done when it develops a golden color, but not amber or burnt.  Remove from heat.  

Stir together crumbs, brown sugar and salt in a bowl.  Stir in browned butter until evenly moistened and combined.  Be sure to scrape in the browned butter bits on the bottom of the pan.  Lightly press crust into bottom and sides of prepared pie pan.  Bake for 8-10 minutes or until set.  Remove from oven to a wire rack to cool while you prepare pie filling. 

To Prepare Filling: 

In a large bowl, whisk together egg yolks and sweetened condensed milk until smooth. Add lime juices, zest and Key lime oil (if using).  Whisk immediately.  Mixture will thicken up.  Pour all into prepared pie crust, smooth out, and bake for 28-32 minutes. The filling will be done when it appears set, but still may have a soft center spot.  Remove to a wire rack to cool before placing into the refrigerator to chill for several hours (or overnight) before serving.  Garnish with sweetened whipped cream if desired.