Preheat your oven to 425°F. Grease four ramekins with butter and dust with cocoa powder to prevent sticking.

In a medium saucepan over low heat, melt the chocolate chips and butter together, stirring until smooth. Remove from heat and whisk in the peanut butter until fully combined.

In a large mixing bowl, combine the powdered sugar, eggs, egg yolks, and vanilla extract. Whisk until well blended.

Gradually add the melted chocolate mixture to the egg mixture, stirring continuously.

Sift the flour and salt together, then fold them into the chocolate mixture until just combined.

Divide the batter evenly among the prepared ramekins. Place them on a baking sheet and bake for 12-14 minutes, or until the edges are firm but the center is still soft.

Remove from the oven and let them cool for 1 minute. Carefully invert each ramekin onto a plate and gently lift to release the cake. Serve immediately, optionally with a scoop of vanilla ice cream or whipped cream.