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1
Preheat oven to 350F.
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2
Spray inside of 9-inch-diameter springform pan with 2 3/4-inch-to 3-inch-high sides with nonstick spray.
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3
Whisk flour, cocoa, and salt in small bowl; set aside.
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4
Combine chocolate and butter in medium metal bowl; place bowl over saucepan of simmering water and stir until melted and smooth.
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5
Remove bowl from over water.
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6
Add both sugars to chocolate mixture and whisk until blended.
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7
Let cool until mixture is barely lukewarm, about 10 minutes.
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8
Whisk egg and vanilla into chocolate mixture.
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9
Fold flour mixture into chocolate mixture.
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10
Spread brownie batter evenly over bottom of prepared pan.
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11
Bake brownie crust until top looks slightly cracked and tester inserted into center comes out with some moist crumbs attached, about 20 minutes.
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12
Transfer pan to rack; cool crust to room temperature, about 30 minutes.
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13
Maintain oven temperature.
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14
Place pan with cooled crust on rimmed baking sheet.
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15
Using electric mixer, beat cream cheese in large bowl until smooth.
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16
Add sugar, flour, lemon peel, and lemon juice; beat until smooth.
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17
Add eggs and yolks, 1 at a time, beating just until incorporated after each addition.
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18
Beat in 1/2 cup sour cream and whipping cream.
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19
Pour filling over brownie crust in pan; smooth top.
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20
Bake cake until puffed, light golden, and set around edges (center will still move slightly when pan is gently shaken), about 1 hour 20 minutes.
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21
Remove cake from oven.
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22
Maintain oven temperature.
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23
Spoon 1 cup sour cream in dollops over top of cake, then spread evenly over top with offset spatula.
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24
Return cake to oven and bake 5 minutes.
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25
Run small sharp knife around sides of cake to loosen.
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26
Place pan with cake directly in refrigerator and chill uncovered overnight (cake may sink in center).
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27
DO AHEAD: Can be made 2 days ahead.
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28
Keep chilled.
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29
Remove pan sides from cake.
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30
Run thin sharp knife between pan bottom and crust to loosen.
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31
Using 2 metal spatulas as aid, transfer cake to platter.
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32
Garnish top edge of cake with chocolate curls and lemon slices.