Line an 18 x 28 x 5 cm (7 x 11 x 2 inch) roasting tin with nonstick baking paper and snip diagonally into the corners so that the paper fits snugly.
Melt half the chocolate and the butter in a heatproof bowl over a pan of gently simmering water. Whisk the eggs, sugar and vanilla extract in a separate bowl with an electric whisk until light and frothy and the whisk leaves a trail when lifted.
Fold the chocolate and butter mixture into the beaten eggs with a metal spoon. Sift the flour and baking powder over the top and then fold in gently. Chop the remaining chocolate and fold half of it into the mixture with half the cranberries.
Pour the mixture into the tin and sprinkle with the remaining chocolate and cranberries. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30–35 minutes until well risen. Leave to cool in the tin.
Lift out of the tin, peel off the paper and cut the cake into squares. The blondies will keep, in an airtight container in a cool place, for a few days.