In a double boiler over simmering water, melt the butter with the chocolate, whisking until smooth.
While the chocolate and butter melts, in a medium bowl, beat the eggs with the egg yolks, sugar, vanilla and salt at high speed until thickened and pale.
Remove the chocolate from the heat and quickly fold it into the egg mixture along with the flour, stirring to combine. Spoon the batter into the prepared ramekins and if using peanut butter, add a teaspoon into the centre of each. Bake for around 10 minutes, until the sides of the cakes are firm but the centers are soft.
Let the cakes cool in the ramekins for 1 minute, then cover each with an inverted dessert plate. Carefully turn each one over, let stand for 10 seconds and then remove the ramekins. Serve immediately with ice cream or whipped cream if desired!