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1
Turn two 15x10-inch baking sheets bottom side up.
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2
Dab butter on corners.
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3
Cover each pan bottom with parchment paper; press paper into butter to anchor.
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4
Stir white chocolate in top of double boiler over barely simmering water until melted.
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5
Spread white chocolate over 1 sheet of parchment.
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6
Stir semisweet chocolate in clean top of double boiler over simmering water until melted.
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7
Spread semisweet chocolate over second sheet of parchment.
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8
Chill both sheets until chocolates are dry but still slightly flexible when edge of parchment is lifted, about 3 minutes for dark chocolate and 5 minutes for white chocolate (do not let chocolate harden completely).
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9
Press heart-shaped cookie cutter about 3 inches long by 3 inches wide into chocolate, forming 12 hearts on each sheet (do not move or lifts hearts).
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10
Chill until firm, about 30 minutes.
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11
Combine both chocolates and butter in small saucepan.
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12
Stir over low heat until smooth.
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13
Cool slightly.
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14
Combine sugar, 2 tablespoons water and corn syrup in heavy small saucepan.
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15
Stir over low heat until sugar dissolves.
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16
Increase heat; boil until candy thermometer registers 238F, tilting pan if necessary to submerge bulb of thermometer and brushing down sides of pan with wet pastry brush, about 4 minutes.
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17
Meanwhile, using electric mixer, beat egg whites and cream of tartar in large bowl until soft peaks form.
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18
Gradually add boiling syrup to whites, beating until firm peaks form and meringue is completely cool, about 5 minutes.
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19
Gradually fold lukewarm chocolate mixture into cooled meringue.
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20
Beat cream and vanilla in small bowl until firm peaks form.
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21
Fold into chocolate mixture.
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22
Chill until cold, about 1 hour.
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23
(Hearts and mousse can be made 1 day ahead.
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24
Cover separately; keep chilled.)
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25
Turn 1 sheet of hearts out onto work surface.
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26
Peel off parchment.
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27
Cut around hearts, if necessary, to free from sheet of chocolate.
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28
Place on baking sheet and chill; transfer trimmings to small saucepan and reserve.
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29
Repeat cutting out hearts from second sheet, reserving trimmings in another small saucepan.
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30
Arrange 12 hearts on baking sheet.
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31
Spread 1/4 cup of mousse over each, leaving 1/2-inch border.
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32
Top each with another chocolate heart.
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33
Refrigerate.
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34
Using fork, stir dark chocolate trimmings over very low heat just until melted.
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35
Using another fork, stir white chocolate trimmings over very low heat just until melted.
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36
Using another fork, stir white chocolate trimmings over very low heat just until melted.
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37
Dip tines of fork into dark chocolate; wave fork over hearts to form pattern.
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38
Repeat with white chocolate.
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39
Chill 1 hour to 8 hours.
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40
Serve cold.
