Melt the bitter and semisweet chocolates and the butter in a heatproof bowl over simmering water, gently stirring occasionally. Bring the cream almost to a boil. Slowly mix the hot cream into the chocolate, stirring until smooth.
You need to treat chocolate very gently, so if melting it over water, the water must never be boiling. The longer you take to melt chocolate the nicer and smoother it will become.
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