Toast pecans in 1 tbsp butter over medium heat 3–4 minutes until fragrant. Cool completely.
Beat softened butter, brown sugar and granulated sugar 2 minutes until creamy.
Beat in egg, egg yolk and vanilla.
Whisk flour, cornstarch, baking soda and salt. Mix into the butter mixture on low just until combined.
Fold in toasted pecans.
Cover and chill dough at least 1 hour.
Heat oven to 350°F (175°C). Scoop 1.5 tbsp balls onto parchment-lined sheets, 2 inches apart.
Bake 10–12 minutes until edges are golden and centers look slightly soft.
Cool on the pan 5 minutes, then move to a rack.