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Place the chopped semisweet and unsweetened chocolate in a 4-quart bowl. Set aside.
In a 1 1/2-quart saucepan, combine the heavy cream and butter over moderate heat and bring to a simmer. Lower the heat to keep the cream hot but not simmering, until needed.
In a 3-quart saucepan, combine the sugar and the lemon juice. Whisk to combine; the sugar will resemble moist sand. Caramelize the sugar for 7 to 8 minutes over moderately-high heat, stirring constantly with a whisk to break up any lumps. The sugar will become clear as it liquefies, then it will brown as it caramelizes. Remove the saucepan from the heat.
Carefully pour about 1/3 of the hot cream into the caramelized sugar. Whisk the caramel until it stops bubbling, then whisk in the remaining cream until smooth. Immediately pour the hot caramel over the chopped chocolate and let stand for 5 minutes, then whisk until very smooth. Serve immediately.