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1
Preheat oven to 325F (160C) degree.
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2
Butter and flour a 13-by 9- inch brownie pan.
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3
Cut 1 cup butter into pieces.
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4
Add 2 cups flour, 1/2 cup sugar, cornstarch, and the butter pieces in a food processor.
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5
Pulse until dough comes together.
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6
Press evenly over the bottom of the prepared pan.
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7
Bake until top feels dry and just starts to brown, 25 to 30 minutes.
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8
Allow to cool completely on a wire rack.
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9
Then turn up the temperature to 350F (180C) degree.
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10
Meanwhile, unwrap caramels and put in a bowl along with the milk, then place the bowl on top of a saucepan with about 1-inch water added over medium heat.
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11
Cook mixture, stirring, until melted and smooth, about 5 minutes.
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12
Spread caramel mixture evenly over cooled shortbread and refrigerate until firm, 25 to 30 minutes.
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13
In the same large bowl set over the saucepan of barely simmering water, stir chocolate and remaining 1/2 cup butter frequently until smooth.
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14
Remove bowl from over water and, with a whisk or a mixer on medium speed, beat in remaining 3/4 cup sugar, eggs, and vanilla until well mixed.
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15
Stir in remaining 1/2 cup flour, baking powder, and salt until well mixed.
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16
Pour chocolate mixture over caramel layer and use a spatula to spread until completely and evenly covering caramel.
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17
Bake in 350F (180C) degree until a wooden stick inserted into the chocolate layer comes out almost clean, about 40 minutes.
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18
Allow to cool completely in the pan on a wire rack.
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19
Cut into squares and serve.
