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1
Adjust oven rack to upper- and lower-middle positions; heat oven to 350 degrees.
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2
Grease three 8-inch pans, line bottom of each pan with parchment paper round, grease agin and flour.
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3
Combine chocolate and cocoa in medium bowl; pour boiling water over and whisk until smooth.
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4
Sift together flours, baking soda, and salt onto large bowl and set aside.
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5
Place butter in bowl of standing mixer and beat at medium-high speed until creamy, about 1 minute.
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6
Add brown sugar and beat on high until light and fluffy, about 3 minutes.
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7
Stop mixer and scrape down bowl.
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8
Increase speed to medium-high and add eggs one at a time, beating 30 seconds after each addition.
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9
Reduce speed to medium; add sour cream and vanilla and beat until combined, about 10 seconds.
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10
Stop mixer and scrape down bowl.
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11
With mixer on low, add about one third of flour mixture, followed by about one half of chocolate mixture.
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12
Repeat, ending with flour mixture; beat until just combined, about 15 seconds.
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13
Do not overbeat.
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14
Remove bowl from mixer; scrape bottom and sides of bowl and mix gently to be certain all ingredients are combined.
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15
Divide batter evenly among cake pans, smooth batter to edges of pan with angled spatula.
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16
If baking three 8-inch cakes, place two pans on lower-middle rack and one on upper-middle rack.
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17
Bake until skewer inserted in center comes out clean, 20 to 23 minutes for 8-inch cakes.
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18
Cool on wire rack 15 to 20 minutes.
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19
Run knife around pan perimeter to loosen.
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20
Invert cakes onto large plate; peel off parchment, and reinvert onto lightly greased rack.
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21
Cool completely before frosting.
