Heat oven to 450°F. In medium saucepan, bring 1 inch water to a simmer. Butter six 6-ounce ramekins and dust with granulated sugar.
Combine chocolate chips and butter in large heatproof bowl and place bowl over (but not in) simmering water to gently melt, stirring occasionally, until smooth. Remove bowl from heat and let cool slightly.
Meanwhile, in medium bowl, whisk together granulated sugar, eggs, yolks, vanilla, and salt until well combined. In small bowl, whisk together flour, cocoa powder, and baking powder. Add egg mixture to chocolate mixture and whisk until combined; add flour mixture and gently whisk until batter is smooth. Divide half of batter among prepared ramekins.
In second small bowl, stir together peanut butter and confectioners’ sugar until combined. Scoop 1 tablespoon into each half-filled ramekin. Cover with remaining chocolate batter, dividing among ramekins. Place on rimmed baking sheet.
Bake until sides of cakes are set and centers are still slightly gooey, 11 to 13 minutes. Let cakes cool 2 minutes, then loosen edges in each ramekin with small offset spatula. Using kitchen towel, carefully invert each onto plate to serve. Dust with additional cocoa powder if desired.
