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1
Bring cream and butter to a simmer in a 3- to 4-quart heavy saucepan, then reduce heat to low.
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2
Whisk in chocolate until smooth and remove from heat.
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3
Transfer ganache to a bowl and chill, covered, stirring every 30 minutes, until thickened but spreadable, about 2 hours.
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4
(If ganache becomes too stiff, let stand at room temperature until slightly softened.)
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5
Preheat oven to 350F.
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6
Butter cake pans and line bottom of each with a round of parchment or wax paper.
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7
Butter paper and dust pans with flour, knocking out excess flour.
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8
Pulse chestnuts with flour, baking soda, and salt in a food processor until finely ground.
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9
Beat butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a standing mixer or about 6 minutes with a handheld.
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10
Add eggs 1 at a time, beating well after each addition, then beat in vanilla (mixture will look a little separated).
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11
Reduce speed to low and add flour mixture and sour cream alternately in 3 batches, beginning and ending with flour and mixing until just combined.
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12
Divide batter evenly among pans and bake in middle of oven until pale golden and springy to the touch, about 30 minutes.
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13
Cool cakes in pans on racks, then invert onto racks and remove parchment.
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14
Melt butter and chocolate in a double boiler or a metal bowl set over a saucepan of barely simmering water, stirring until smooth, and stir in Cognac.
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15
Transfer to a bowl and chill, covered, stirring occasionally, until thickened to the consistency of softened butter, about 1 hour.
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16
(If mixture becomes too stiff, let stand at room temperature until softened.)
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17
Beat cream in a bowl with cleaned beaters until it just holds soft peaks, then chill, covered, while beginning to assemble torte.
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18
Put 1 cake layer on cardboard round on a rack set in a shallow baking pan (1 inch deep).
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19
Spread 1/2 cup ganache evenly over top of layer and sprinkle with all of chopped marrons glaces.
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20
Top marrons glaces with another 1/4 cup ganache and cover with another cake layer.
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21
Beat egg whites with a pinch of salt using cleaned beaters at medium-high speed until they just hold soft peaks.
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22
Add sugar and beat at high speed until whites just hold stiff peaks.
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23
Stir whipped cream into chocolate Cognac mixture, then stir in one third of whites to lighten.
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24
Fold in remaining whites gently but thoroughly.
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25
Spoon mousse immediately onto cake layer (it sets quickly), spreading evenly, then top with third cake layer.
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26
Chill torte, covered, until mousse layer is firm, about 1 hour.
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27
Keep remaining ganache at a spreadable consistency at room temperature, chilling, covered, if it becomes too soft.
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28
Reserve 1 1/4 cups ganache in a metal bowl, then spread remainder over top and side of torte to seal in crumbs.
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29
Chill until firm, about 1 hour.
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30
Set bowl of reserved ganache over a saucepan of barely simmering water, stirring until ganache reaches a pourable consistency.
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31
Remove bowl from heat and cool 5 minutes.
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32
Pour ganache evenly over top of torte, making sure it coats all of side.
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33
Shake rack gently to smooth glaze (let excess drip into baking pan).
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34
Transfer cake on cardboard to a cake stand or plate using 2 large heavy metal spatulas and chill until set.
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35
Garnish just before serving.
