Line 6 bun tin holes with paper cases.
Melt the plain chocolate chips and butter together in a small saucepan over a low heat. Beat the eggs, sugar, flour and cocoa powder together in a bowl. Fold in the melted chocolate mixture and the white chocolate chips.
Spoon the mixture into the paper cases and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 12 minutes until risen and firm to the touch. Transfer to a wire rack to cool slightly. Serve warm.