Chocolate-Hazelnut Shortbread Squares

Crust: Preheat oven to 375 degrees with rack in middle. Butter a 9-inch square cake pan. Line bottom and 2 sides with parchment, leaving a 1-inch overhang. Butter parchment.

With a mixer on medium speed, beat butter with brown sugar, vanilla, and salt until fluffy, about 2 minutes. Reduce speed to low; beat in flour until combined and a soft dough forms.

Spread dough in prepared pan, pressing evenly with the bottom of a dry-measure cup. Prick all over with a fork. Bake until golden, about 15 minutes. Let cool completely in pan on a wire rack.

Truffle: In a small saucepan, bring cream just to a boil over medium heat. Remove from heat; whisk in chocolate until melted and combined. Add liqueur and salt, whisking until blended, then stir in nuts. Let cool slightly.

Pour chocolate mixture evenly over cooled crust. Refrigerate, uncovered, until cold and set, about 2 hours.

Remove shortbread from pan using parchment; place on a cutting board. Dust top completely with cocoa powder. Cut into 48 squares, dipping knife in hot water and wiping clean before each slice. Refrigerate in a single layer until ready to serve, up to 1 week.