Warm the rum, add the raisins and leave to soak for 2 hours or overnight.
Heat the dark chocolate and butter gently in a saucepan until both have melted. Meanwhile, whisk the eggs and sugar together in a bowl, using an electric whisk, until very thick and the whisk leaves a trail when lifted above the mixture.
Fold the warm chocolate and butter into the whisked eggs and sugar. Sift the flour and baking powder over the top then fold in. Pour the mixture into an 18 x 28 cm (7 x 11 inch) roasting tin lined with nonstick baking paper, and ease into the corners. Spoon the rum-soaked raisins over the top.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes until well risen, the top is crusty and cracked and the centre still slightly soft. Leave to cool and harden in the tin.
Lift out of the tin using the lining paper. Melt the milk chocolate in a heatproof bowl set over a saucepan of gently simmering water then drizzle over the top of the brownies. Leave to harden then cut into 20 pieces. Peel off the paper and store in an airtight tin for up to 3 days.