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1
Preheat oven to 350F.
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2
Beat together cream cheese and peanut butter in a bowl with an electric mixer until smooth.
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3
Add yolks, sugar, and flour and beat until just combined.
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4
Transfer to a pastry bag and chill.
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5
Put cocoa powder in a bowl and add boiling water in a stream, whisking until smooth.
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6
Stir in chopped chocolate and let stand 1 minute, then whisk until smooth.
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7
Cool to room temperature, then whisk in sour cream and vanilla.
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8
Whisk together flour, baking soda, and salt.
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9
Beat together butter and brown sugar in a large bowl with cleaned beaters until pale and fluffy, then add eggs 1 at a time, beating after each addition until just combined.
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10
Reduce speed to low and add flour mixture and cocoa mixture alternately in 3 batches, beginning and ending with flour mixture and beating until batter is just combined.
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11
Spoon some of chocolate mixture into second pastry bag and pipe into well-buttered cake molds to fill each about one fourth of the way.
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12
Refill pastry bag as needed.
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13
Form 3 evenly spaced, equal pockets of peanut butter mixture in each cake by placing tip of pastry bag with peanut butter mixture onto bottom of mold and squeezing about a heaping teaspoon each time.
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14
Fill molds three-fourths full with remaining chocolate mixture, smoothing tops but being careful not to disturb peanut butter mixture.
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15
Bake cakes in upper and lower thirds of oven, switching position of pans halfway through baking, until a tester comes out with crumbs adhering, 15 to 18 minutes (cakes will be dark).
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16
Run a thin knife around inside edges of molds and gently invert cakes onto racks, then cool completely.
