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1
Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish, then line with parchment paper, leaving an overhang on 2 sides; butter the parchment.
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2
Beat 2 1/2 sticks butter, the granulated sugar and 1 teaspoon vanilla in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 5 minutes.
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3
Reduce the mixer speed to low; add the flour and beat until just incorporated.
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4
Transfer the dough to the prepared baking dish and set a piece of plastic wrap directly on the surface.
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5
Press into an even layer, then peel off the plastic.
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6
Bake until golden brown, 30 to 35 minutes.
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7
Transfer to a rack to cool slightly, then remove from the pan using the parchment.
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8
Cut into triangles.
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9
Make the caramel: Combine the remaining 1/2 stick butter, the brown sugar, corn syrup, heavy cream, espresso powder and kosher salt in a small saucepan over medium heat.
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10
Cook, stirring occasionally, until a candy thermometer registers 235 degrees F, about 6 minutes.
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11
Remove from the heat and stir in the remaining 1 teaspoon vanilla and the vinegar.
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12
Let cool 5 minutes, then drizzle over the shortbread.
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13
Sprinkle with sea salt.
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14
Photograph by Andrew Purcell
