Preheat oven to 350°F. Line a 9x13-inch pan with foil and spray with nonstick cooking spray.
Place pecans in a small frying pan and heat over medium low heat, stirring often, until toasted, about 3-5 minutes. Don’t walk away or they’ll burn – as soon as you smell them they’re done. Remove from pan to a plate to cool completely.
Cream butter and sugar in a large bowl with an electric mixer. Mix in vanilla, salt, then cornstarch. Slowly mix in flour until combined, then stir in toasted pecans.
Press dough into a thin layer in prepared pan. Bake for 25-30 minutes for soft-baked or up to 35 minutes for crunchy.
Cool completely before slicing into bars. Store in an airtight container for up to 4 days or freeze for up to 3 months.