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1
Preheat an oven to 350 degrees F.
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2
In a metal bowl, combine both chocolates with the butter.
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3
Put the bowl over a pot of boiling water for a makeshift double boiler.
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4
Lower the heat so the water is hot but not boiling.
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5
Stir the chocolate with a plastic spatula, from time to time, until the chocolate and butter melt together.
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6
Set aside.
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7
In a large bowl, whisk together the egg, sugar and vanilla.
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8
Beat until the mixture comes together and becomes a pale yellow color, about 1 minute.
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9
Combine the flour, baking powder and salt in a separate bowl.
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10
Sift and add it to the egg mixture.
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11
Stir until blended.
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12
Using a rubber spatula, add half of the melted chocolate to the batter and gently stir to combine.
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13
Add the remaining chocolate and stir again until combined.
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14
Drop tablespoonfuls of the cookie batter onto a greased baking sheet or a greased parchment-lined baking sheet.
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15
Leave room in between the cookies so they can spread out as they bake.
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16
You should be able to drop 18 cookies total, on the baking sheet, depending on their size.
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17
Put the baking sheet in the center of the oven and bake for 4 minutes.
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18
Rotate the pan halfway and bake for an additional 4 minutes.
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19
Remove the pan from the oven and allow the cookies to cool, 10 to 15 minutes, before transferring to a plate.
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20
Preheat the oven to 250 degrees F.
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21
Put the chopped white chocolate into a baking dish, preferably not a metal dish, and melt it in oven.
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22
Remove it after about 10 minutes and stir.
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23
Put back into the oven and continue to cook until caramelized, about 10 minutes.
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24
Meanwhile, whisk together the mascarpone and rum in a small bowl.
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25
Add the caramelized chocolate to a medium bowl.
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26
Blend in the mascarpone mixture until combined.
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27
Refrigerate until the cookies have cooled.
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28
Bring the filling to room temperature before spreading it between the cookies to make sandwiches.
