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1
Adjust oven rack to middle position; heat oven to 325 degrees.
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2
For the Brownie Base Layer:
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Cut parchment paper to fit bottom of a 9-inch springform pan & spray with cooking spray.
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4
Whisk to combine flour, salt, & baking powder in medium bowl; set aside.
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In microwave-safe bowl heat butter & chocolate on high for 45 seconds, then stir and heat for 30 seconds more. Stir again, if necessary, repeat in 15-second increments until melted; do not let chocolate burn.
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6
When chocolate mixture is smooth gradually whisk in sugar. Add eggs one at a time, whisking after each until thoroughly combined. Whisk in vanilla. Add flour mixture in two additions, folding with spatula until batter is completely smooth and mixed.
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Transfer batter to prepared pan; using a spatula to spread batter evenly and smooth. Bake until toothpick inserted into center of brownies comes out with a few moist crumbs attached, 25-30 minutes. Cool on wire rack to room temperature, about 1 hour (do not remove the springform ring).
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8
For the Cake Layer:
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Line bottom of 9-inch round cake pan with parchment & spray with cooking spray.
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10
Combine chocolate, cocoa powder, & hot water in a medium microwave-safe bowl & heat until melted; about 45 seconds. If needed, heat in additional 15 second intervals until melted. (Do not burn) Add 1/4 cup sugar to chocolate mixture and stir until thick and glossy, 1 to 2 minutes. Set aside to cool.
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Whisk flour, baking soda, and salt in medium bowl. Set aside. Combine buttermilk and vanilla in small bowl and set aside.
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In bowl whisk eggs and yolks on medium-low speed until combined, about 10 seconds. Add remaining sugar, increase speed to high, and whisk until fluffy and lightened in color, 2 to 3 minutes.
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Add cooled chocolate mixture to egg/sugar mixture and mix on medium speed until thoroughly mixed, 30 to 45 seconds, scrape down sides of bowl with spatula as needed. Add softened butter 1T. at a time, mixing about 10 seconds after each addition. Add about 1/3 of flour mixture followed by 1/2 of buttermilk mixture, mixing until incorporated after each addition (about 15 seconds).
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Repeat using 1/2 of remaining flour mixture & all of remaining buttermilk mixture (batter may appear separated). Scrape down sides of bowl & add remaining flour mixture; mix at medium-low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer & fold batter once or twice with spatula to incorporate any remaining flour. Pour into prepared pan & smooth.
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Bake until toothpick inserted into center comes out with a few crumbs attached, about 30 - 40 minutes. Cool cake in pan 15 minutes, then invert onto wire rack and cool completely.
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Chocolate Mousse Filling:
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Place stand mixer bowl and whisk attachment in the freezer or fridge.
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Chop the chocolate and place in medium bowl. Heat 1/2 cup of the heavy cream in microwave-safe cup until it begins to boil. Pour hot cream into bowl with chopped chocolate and whisk until smooth. Let cool for about 15 minutes.
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Once chocolate mixture is cooled, place remaining heavy cream(1 1/4 cup plus 2 T) and sugar in the well-chilled bowl of an electric mixer fitted with a well-chilled whisk attachment. Whisk on high until stiff peaks form, about 1 1/2 minutes. Remove the bowl from the mixer.
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20
Gently fold in chocolate mixture with the whipped cream mixture.
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21
Ganache:
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Heat the heavy cream and the butter in a microwave-safe bowl. Bring to a boil.
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Place the chopped semisweet chocolate in a 3-quart stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Ganache should be brought to room temperature (about 40 minutes). There will be ganache left-over after icing cake. You could chill about 1 cup to use for additional decorating if desired.
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To Assemble:
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Spread mousse over top of the brownie base evenly. Use an offset spatula or bench scraper to smooth the top.
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Place cake round over mousse, pressing down lightly. Chill for 1 hour.
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27
Remove springform ring and parchment collar. Use a spatula to smooth the room temperature ganache over the cake top and sides, covering evenly.
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28
Decorate with chocolate shavings on the sides if desired and with the additional chilled ganache if desired.
