Beat together cream cheese and peanut butter in a bowl with an electric mixer until smooth.
Add yolks, sugar, and flour and beat until just combined.
Transfer to a pastry bag and chill.
Put cocoa powder in a bowl and add boiling water in a stream, whisking until smooth. Stir in chopped chocolate and let stand 1 minute, then whisk until smooth. Cool to room temperature, then whisk in sour cream and vanilla.
Whisk together flour, baking soda, and salt. 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. 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.
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. Refill pastry bag as needed.
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.
Fill molds three-fourths full with remaining chocolate mixture, smoothing tops but being careful not to disturb peanut butter mixture.
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).
Run a thin knife around inside edges of molds and gently invert cakes onto racks, then cool completely.
• Chocolate and peanut butter mixtures can be spooned, instead of piped, into molds; don’t worry if the peanut butter dollops are not perfectly uniform in size. • Cakes may be made 2 days ahead and kept in 1 layer in an airtight container at room temperature.
