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1
Preheat oven to 350u00b0. Line bottoms of three 6-in. round baking pans with parchment paper; grease and flour parchment and sides of pans.
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2
In a large bowl, whisk flour, sugar, baking powder and salt. Beat in butter until crumbly. Add egg whites, one at a time, beating well after each addition. Gradually beat in 1/4 cup milk and extracts; beat on medium until light and fluffy, about 2 minutes. Gradually beat in remaining milk. Gently fold in jimmies.
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3
Transfer batter to prepared pans. Bake until a toothpick inserted in center comes out clean, 25-30 minutes. Cool in pans 10 minutes before removing to wire racks; remove paper. Cool completely.
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4
For buttercream, in a microwave, melt chocolate with cream until smooth, stirring every 30 seconds. Set aside to cool slightly. In a heatproof bowl of a stand mixer, whisk egg whites, sugar, cream of tartar and salt until blended. Place over simmering water in a large saucepan over medium heat. Whisking constantly, heat mixture until a thermometer reads 160u00b0, 8-10 minutes.
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5
Remove from heat. With whisk attachment of stand mixer, beat on high speed until cooled to 90u00b0, about 7 minutes. Gradually beat in butter, a few tablespoons at a time, on medium speed until smooth; beat in melted chocolate mixture and vanilla extract. Gradually beat in cocoa through a sifter.
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6
Spread frosting between layers and over top and sides of cake. Reserve remaining buttercream for decorating. Decorate as desired. Store in refrigerator.
