How to Make It
Preheat the oven to 350°. Line a 9-by-13-inch pan with parchment paper, allowing 1 inch of overhang on the long sides. In the bowl of a stand mixer fitted with the paddle, mix the butter with the flour, sugar and salt at medium speed until a crumbly dough forms, about 6 minutes. Press the dough into the bottom of the prepared pan in an even layer. Refrigerate for 20 minutes.
Bake the shortbread for 25 minutes, until golden brown; rotate the pan from front to back halfway through baking. Transfer to a rack and let cool completely.
In a medium saucepan, melt the butter over moderate heat. Whisk in the honey, both sugars and salts and the vanilla. Cook, stirring occasionally, until the caramel reaches 220° on a candy thermometer, 2 to 3 minutes. Carefully add the cream and cook until the mixture reaches 238°, 3 to 4 minutes. Stir in all of the walnuts and cook until fragrant and toasty and slightly thickened, about 3 minutes longer.
Pour the walnut caramel over the shortbread and let cool completely. Using the parchment paper, lift onto a cutting board. Cut into bars and serve.