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1
Preheat oven to 350F and butter 2 (15- by 10- by 1-inch) baking pans.
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2
Line bottom and sides of each pan with a large sheet of wax paper and butter paper.
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3
Dust pans with flour, knocking out excess.
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4
Melt chocolate in a double boiler or a small metal bowl set over a small saucepan of barely simmering water, stirring occasionally.
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5
Remove from heat.
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6
Beat together butter and 1/2 cup sugar in a large bowl with an electric mixer until light and fluffy and beat in yolks, 1 at a time, beating well after each addition.
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7
Beat in chocolate.
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8
Sift in flour, cocoa, and salt and beat on low speed until combined well.
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9
Beat egg whites with cream of tartar in another bowl with clean beaters until they hold soft peaks, then add remaining 1/4 cup sugar, beating until whites just hold stiff peaks.
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10
Stir one fourth of whites into batter to lighten and fold in remaining whites gently but thoroughly.
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11
Divide batter between pans (about 2 1/4 cups per pan) and carefully spread evenly.
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12
(Layers will be thin.)
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13
Bake in upper and lower thirds of oven, switching position of pans halfway through baking, until cake is set and firm to touch, 14 to 18 minutes total.
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14
Cool layers in pans on racks 10 minutes before inverting racks over pans and flipping layers onto them.
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15
Remove wax paper carefully and discard.
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16
Spread jam evenly over 1 warm layer and cool layers completely.
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17
Transfer jam-coated layer to a baking sheet or tray lined with a sheet of parchment or wax paper.
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18
Bring cream just to a boil and slowly pour over chocolate in a bowl.
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19
Stir until smooth and pour over plain cake layer, spreading to coat top evenly.
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20
Let stand in a cool place until set, about 1 hour.
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21
Bring cream just to a boil and slowly pour over chocolate in a large metal bowl.
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22
Stir until smooth and set bowl in an ice bath.
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23
Stir occasionally until cold.
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24
Remove from ice bath and beat with an electric mixer until mousse just holds soft peaks.
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25
(If mousse becomes grainy, melt over a saucepan of barely simmering water and repeat chilling and whipping.)
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26
Quickly spread evenly over jam layer (mousse will stiffen as it stands) and chill while making whipped-cream filling.
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27
Sprinkle gelatin over water in a small metal bowl and let soften 1 minute.
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28
Put bowl over a small saucepan of boiling water and heat, stirring occasionally, until gelatin is dissolved.
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29
Remove pan from heat but keep bowl on pan.
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30
Beat cream, confectioners sugar, and vanilla with an electric mixer until it holds a soft shape and beat in warm gelatin mixture.
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31
Continue beating until cream just holds stiff peaks, then spread evenly over top of mousse-coated layer.
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32
Cut glazed layer lengthwise into thirds and crosswise into tenths and reassemble bars on top of cream filling.
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33
Chill cake, uncovered, until glaze is firm, about 1 hour, then cover with plastic wrap and chill until ready to serve.
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34
Just before serving, cut cake with a large knife, wiping it off with a hot damp cloth between cuts.
