Break half the chocolate into pieces, place in a saucepan with the butter and heat gently until melted. Dice the remaining chocolate.
Whisk the eggs, sugar and vanilla together in a bowl, using an electric whisk, for about 5 minutes until very thick and foamy and the whisk leaves a trail when lifted above the mixture. Fold in the melted chocolate mixture and then the flour and baking powder. Gently fold in half the chopped chocolate and apricots.
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. Sprinkle with the remaining chocolate and apricots. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes until well risen, the top is crusty and the centre still slightly soft.
Leave to cool in the tin then lift out using the lining paper and cut into 20 small pieces. Peel off the paper and store in an airtight tin for up to 3 days.