Preheat the oven
to 180'C/350'F /Gas 4. Line a 25 x 10cm/10 x 4in
ring mould with greaseproof paper and grease the
paper. Dust with flour. Melt the chocolate in the
top of a double boiler, stirring occasionally. Set
aside.
Sift together the
flour and baking powder. In another bowl, cream the
butter, sugar and vanilla essence until light and
fluffy. Add the eggs, two at a time, then gradually
blend in the flour mixture.
Spoon half of the
mixture into the ring mould. Stir the chocolate into
the remaining mixture, then spoon into the tin. With
a palette knife, swirl the mixtures for a marbled
effect.
Bake until a
skewer inserted in the centre comes out clean, about
1 hours. Cover with foil halfway through baking.
Stand for 15 minutes, then unmould and transfer to a
wire rack. To serve, dust with icing sugar.