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Preheat the oven to 180°C/350°F/Gas mark 4.
Grease a 20cm/8in spring form cake tin.
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To make
the cake, melt the chocolate in a heatproof bowl
set over a pan of
simmering water. Remove from the heat and allow
to cool slightly.
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Beat the butter with 1 tablespoon sugar until
creamy, then add the egg yolks a
little at a time.
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Add the melted chocolate, then
sift the flour and baking powder.
Beat the egg whites with the salt until stiff
and then add the remaining sugar.
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Fold gently
into the mixture and pour into the prepared tin. Bake for 40 minutes, then turn out on to a wire
rack to cool.
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To make the truffle filling, melt
the chocolate as before and leave to cool
slightly. Cream the butter and sugar until pale
and thick.
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Add half the orange juice and all the
zest. Mix in the chocolate and beat until fluffy.
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Cut the cake in half horizontally and sprinkle
the remaining orange juice and the
liqueur on the cut side of the bottom half.
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Spread the lower layer with half of the
truffle filling and place the other layer on
top. Spread the rest of the truffle filling over
the top. Chill for 2 hours before serving.