Preheat the oven to 200°C (180°C for fan assisted ovens)/Gas Mark 6. Place a baking sheet in the oven to heat up. Generously grease 8 x 175ml individual pudding basins or 8 holes in a deep 12-hole muffin tray (see tip).
In a medium-sized microwave-proof bowl, microwave the chocolate chips and butter, uncovered, on High for 45 seconds-1 minute 15 seconds, or until melted. Stir until smooth.
Beat the eggs and egg yolk in a large bowl with a balloon whisk or electric mixer until foamy. Gradually beat the dry brownie mix into the egg mixture until well blended – the mixture will be quite thick. Gently stir in the melted chocolate and butter. Divide the brownie mixture evenly between the greased pudding basins or muffin holes.
Bake on the hot baking sheet for 14-17 minutes, or until the top and edges are set and when the puddings are gently pressed with your fingertips you should feel a slight wobble. The centres will be soft (if you have a food thermometer the internal temperature should be at least 71°C). Leave to stand for 5 minutes.
Loosen each pudding by running a knife around the edges. If baked in a muffin tray, turn the whole tray upside down onto a heatproof tray or baking sheet to release the puddings, then carefully transfer to serving plates. If baked in pudding basins, turn each one out onto a serving plate. Serve immediately with a spoonful of crème fraîche and decorate with raspberries and a scattering of heart-shaped sprinkles, if you like.