Preheat the oven to 180°C/Gas Mark 4. Line a 18x28cm (7x11in) roasting tin with a large piece of non- stick baking paper, snipping diagonally into the corners so that the paper fits snugly over the base and up the sides of the tin.
Break the chocolate into a bowl, add the butter and set the bowl over a saucepan of gently simmering water and leave until melted.
Whisk the eggs, sugar and vanilla in a second bowl with an electric whisk for about 5 minutes until very thick. The mixture will leave a trail when the whisk is lifted up.
Gently fold the melted chocolate and butter into the egg mixture, then add the flour and baking powder. Pour the mixture into the lined tin and sprinkle with the raspberries.
Bake for 30-35 minutes until well risen, the top is crusty and the centre still a little soft. Leave to cool in the tin, then lift the paper and blondies out of the tin and put onto a chopping board. Drizzle with the melted chocolate to decorate, then leave to set. Cut into squares and peel off the paper to serve.