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1
Preheat oven to 375F.
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2
Butter a 9-inch square baking pan and line bottom with wax paper.
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3
Chop chocolate into small pieces.
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4
In a double boiler or a metal bowl set over a saucepan of barely simmering water melt chocolate and butter, stirring until smooth.
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5
Remove top of double boiler or bowl from heat and whisk sugar into chocolate mixture.
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6
Whisk in eggs 1 at a time until combined well.
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7
Sift flour and cocoa powder over chocolate mixture and whisk until just combined.
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8
Pour batter into baking pan and bake in middle of oven until a tester comes out clean, about 20 minutes.
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9
Cool cake completely in pan on a rack and invert onto a baking sheet lined with wax paper.
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10
Chop chocolate.
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11
In a dry heavy saucepan cook sugar over moderate heat, without stirring, until it begins to melt.
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12
Continue to cook sugar, stirring with a fork, until a deep-golden caramel.
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13
Remove pan from heat and add cream (mixture will bubble up and steam).
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14
Simmer mixture, stirring, until caramel is dissolved.
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15
Remove pan from heat and add chocolate, stirring until mixture is smooth.
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16
Pour ganache over top of cake and smooth with a spatula.
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17
Chill cake, uncovered, at least 30 minutes and, covered, up to 3 days.
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18
Lightly grease a baking sheet.
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19
In a dry small heavy saucepan cook sugar over moderately low heat, stirring slowly with a fork (to help sugar melt evenly), until a pale-golden caramel.
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20
Continue to cook caramel, without stirring, gently swirling pan, until golden.
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21
Remove pan from heat and pour caramel onto baking sheet.
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22
Cool caramel completely.
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23
Pry caramel from baking sheet with your fingers and in a food processor pulse caramel until coarsely ground.
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24
Sprinkle the ground caramel evenly over top of cake and with a sharp knife cut cake into 1 1/4-inch diamonds.
