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1
Preheat the oven to 350 degrees F. Line a 9 by 13-inch baking pan with parchment or waxed paper.
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2
In a mixer fitted with a paddle attachment (or using a hand mixer), cream 8 ounces butter until soft.
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3
Add 1/2 cup of the sugar and mix until incorporated.
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4
In a separate bowl, stir together the flour and cornstarch.
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5
Add the dry ingredients to the butter mixture and mix at low speed just until the ingredients are incorporated and the dough comes together.
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6
Turn the dough onto a floured work surface and knead it 5 to 10 times, to bring the dough together and smooth it out.
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7
Re-flour your work surface.
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8
With a rolling pin, roll the dough out to fit the sheet pan.
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9
To transfer to the sheet pan, roll the dough up onto the rolling pin, lift it up, and unroll into the pan.
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10
Using light strokes of the rolling pin, roll the dough into the corners and edges of the pan, and roll out any bumps (or press the rolled-out dough thoroughly into the pan with your fingers).
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11
Prick the shortbread all over with a fork to prevent any buckling or shrinking.
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12
Bake in the center of the oven for 15 minutes.
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13
After 15 minutes, rotate the pan and knock it once against the oven rack, to ensure even cooking and a flat surface.
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14
Bake 10 to 15 minutes more, until very lightly browned.
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15
Let cool in the pan.
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16
Pour the remaining 2 1/2 cups sugar into the center of a deep saucepan.
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17
Carefully pour the water around the sugar, trying not to splash any sugar onto the sides of the pan.
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18
Do not stir; gently draw your finger through the center of the sugar twice, making a cross, to moisten it.
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19
Over medium-high heat, bring to a boil without stirring.
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20
Reduce the heat to a fast simmer and cook without stirring until amber-caramel in color, 10 to 20 minutes.
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21
Immediately remove from the heat and stir in 2 tablespoons butter with a wooden spoon.
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22
Slowly and carefully pour in the lukewarm cream, stirring slowly but constantly (it will bubble up and may splatter).
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23
Pour over the baked shortbread and smooth the top.
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24
Place in the refrigerator, uncovered, to harden slightly.
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25
When the caramel has set, melt the chocolate in the top of a double boiler (or in a mixing bowl) set over barely simmering water, stirring frequently.
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26
Stir in the vegetable oil (this will make the chocolate less brittle when it hardens).
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27
Pour over the cooled caramel and spread quickly with the back of a spatula or spoon to cover the entire surface.
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28
Let cool in the pan.
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29
With a heavy knife, cut into 1 1/2-by-3-inch bars.
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30
Store in an airtight container.
