Spray a 9 x 5 inch loaf pan with cooking spray and line with parchment paper. Set aside. Melt butter in a heavy bottomed 2 quart saucepan over medium-low heart, stirring as needed.
1 cup unsalted butter
Stir in the brown sugar and cook for 2 minutes, stirring constantly.
2 cups packed brown sugar
Gently pour in the milk and stirring constantly, bring mixture to a boil.
½ cup whole milk
Remove from heat and let cool for 30 to 40 minutes, or until the mixture is lukewarm or at room temperature.
Stir in the vanilla extract.
1 teaspoon vanilla extract
Stir in powdered sugar, one to one-half cup at a time, stirring until completely combined. Add powdered sugar just until the mixture is thick and has the consistency of fudge. (It wall harden as it chills.)
3 ½ to 4 cups sifted powdered sugar
Stir in coarsely chopped pecans.
1 ½ cups coarsely chopped pecans
Spoon into the prepared loaf pan and top with additional pecans if desired.
Cover and cool until firm.
Cut into squares and store in the refrigerator in an airtight container.