Preheat oven to 350°F. Line 2 baking sheets with parchment paper.
In a large bowl of the hand mixer, beat butter, peanut butter, and brown sugar until fluffy, 2–3 minutes.
Add egg and vanilla. Mix until combined, scraping the sides.
In another bowl, whisk flour, cornstarch, and salt.
Add dry mix to wet ingredients. Stir until just combined.
Scoop dough onto prepared trays, leaving space between each.
Press a small well into each dough ball. Fill each with a small dollop of fluff.
Sprinkle with crushed nuts.
Bake for 10–12 minutes, or until edges are lightly golden.
Cool on tray for 5 minutes, then transfer to wire rack.
