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1
Make the shortbread Preheat the oven to 350.
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2
Line a 9-by-13-inch pan with parchment paper, allowing 1 inch of overhang on the long sides.
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3
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.
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4
Press the dough into the bottom of the prepared pan in an even layer.
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5
Refrigerate for 20 minutes.
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6
Bake the shortbread for 25 minutes, until golden brown; rotate the pan from front to back halfway through baking.
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7
Transfer to a rack and let cool completely.
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8
Meanwhile, make the walnut caramel In a medium saucepan, melt the butter over moderate heat.
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9
Whisk in the honey, both sugars and salts and the vanilla.
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10
Cook, stirring occasionally, until the caramel reaches 220 on a candy thermometer, 2 to 3 minutes.
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11
Carefully add the cream and cook until the mixture reaches 238, 3 to 4 minutes.
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12
Stir in all of the walnuts and cook until fragrant and toasty and slightly thickened, about 3 minutes longer.
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13
Pour the walnut caramel over the shortbread and let cool completely.
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14
Using the parchment paper, lift onto a cutting board.
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15
Cut into bars and serve.
