Preheat oven to 425 degrees then line a half sheet pan with foil in case of spills then set aside. Spray two, 6oz ramekins or custard cups with nonstick spray then set aside. Whisk together the whole egg plus egg yolk in a small bowl then set aside.
Add butter and chocolate chips to a medium-sized bowl then microwave for 30 seconds. Stir then microwave in 10 second increments, stirring between increments, until chocolate is smooth and melted. Add powdered sugar then stir to combine. Add whisked eggs and vanilla then stir to combine. Add flour and salt then stir to combine. Divide batter between prepared cups (about 1/2 cup batter each) then scoop 1 heaping teaspoon peanut butter into the middle of each cup. Use your finger to help sink the peanut butter into the center of the batter.
Place cups onto sheet pan then bake for 12 minutes or until tops are soft but not jiggly. Let lava cakes cool for 10 minutes then overturn onto plates and serve with fresh berries.
Notes
The longer the cakes sit, the more the "lava" firms up. I like a 10 minute resting time before overturning.