- 3/4 cup cream cheese (6 oz), softened
- 1 1/4 cups chunky peanut butter (12 oz)
- 6 large egg yolks
- 2/3 cup granulated sugar
- 1/4 cup all-purpose flour
- 1 1/4 cups unsweetened cocoa powder (not Dutch-process)
- 1 1/4 cups boiling water
- 3 1/2 oz fine-quality bittersweet chocolate (not unsweetened), finely chopped
- 1 (8-oz) container sour cream
- 1 tablespoon vanilla
- 2 cups all-purpose flour
- 1 3/4 teaspoons baking soda
- 3/4 teaspoon salt
- 2 1/2 sticks (1 1/4 cups) unsalted butter, softened
- 2 cups packed light brown sugar
- 3 large eggs
- Special equipment: 2 large pastry bags with 3/4-inch openings (or large heavy-duty sealable plastic bags with 1 corner snipped off) and 3 mini angel-food-cake pans (each with molds for six 6- by 3 1/2- by 1 3/4-inch cakes) or large-muffin pans (1 cup)
- Accompaniment: vanilla ice cream or whipped cream
- Preheat oven to 350°F.
- 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.