Preheat the oven to gas 6, 200°C, fan 180°C. Generously grease 4 x 175ml pudding moulds with butter. These can be standard dariole moulds or rum baba tins, which are narrower but with higher edges, which will give a taller pudding.
Melt the butter, margarine and chocolate in a heatproof bowl over a saucepan of simmering water, stirring occasionally, until liquid and smooth. Alternatively, microwave on high in 30-sec bursts, stirring between each one, for 2-3 mins, until melted. Stir in the vanilla extract and set aside to cool.
In a separate bowl, use an electric whisk to mix the sugar, eggs and yolks, until pale and very thick. This will take about 4 mins, or 6-7 mins by hand. Then fold in the flour.
Beat in the cooled chocolate mix, then divide between the moulds and press ½ tbsp peanut butter into the centre of each. There should be about 1cm of space between the chocolate and the rim.
Bake for 12 mins in the dariole moulds, or 10 mins in the rum baba tins. Remove from the oven and leave to stand for 40 secs before inverting onto serving plates. Serve immediately, with a scoop of crème fraîche or vanilla ice cream.
