In a medium-sized bowl, add dark chocolate and butter, and microwave at 15-second intervals, stirring after each interval until fully melted.
Preheat oven to 425˚F (220˚C).
In a medium mixing bowl, whisk together eggs, sugar and vanilla, then pour in the chocolate mixture.
Grease the inside of 4 ramekins with butter and lightly dust with cocoa powder.
Place one ¼ cup (60 ml) scoop of batter into each ramekin, then follow with a small scoop of peanut butter (about 2 tbsp) in the center of each ramekin. Finish with another ¼ cup (60 ml) scoop of the batter.
Bake at 425˚F (220˚C) for 10-12 minutes, or until sides are firm and center is soft.
Using a kitchen towel to hold the ramekin, place the serving plate on top of the ramekin and turn the lava cake out onto the plate (gently shaking the ramekin if needed) until the cake comes out.