Heat oven to 350°F. Line a 9-inch square pan with parchment paper and spray with cooking spray.

In a medium bowl, mix together 2/3 cup butter, white sugar, and flour until evenly crumbly. Press into the lined 9-inch square baking pan. Bake for 20 minutes or until light golden brown. Remove from oven but keep it on. Let the crust cool while you prepare the caramel cashew layer.

Place sugar in a nonstick saucepan over medium-high heat. Let stand until the bottom layer of sugar begins to melt, then start stirring, continuing to stir until sugar has turned light brown in color and smooth in texture.

Remove the pan from the heat and slowly pour in the heavy cream, being careful of it splattering. Stir constantly until the mixture is smooth, about 3 to 5 minutes. Stir in the butter, vanilla extract, and salt.

Quickly stir in the cashews while the caramel is still hot. Pour the mixture into the pan over the crust. Bake for 15 to 20 minutes or until the caramel is all bubbling.

Let cool completely before cutting. Cut into 16 bars.

Store in an airtight container at room temperature for up to 2 days, in the refrigerator for up to 1 week, or in the freezer for up to 3 months.