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1
Preheat oven to 350F.
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2
Lightly butter 2 (8-inch) round cake pans and dust with flour, knocking out excess.
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3
Sift together flour, baking soda, baking powder, and salt.
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4
Beat together butter and sugar in mixer at high speed until lightened, about 5 minutes.
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5
Add yolks 1 at a time, beating well after each addition, then beat in vanilla.
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6
Add half of flour mixture and mix at low speed until just blended.
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7
Add buttermilk, mixing until just combined, then add remaining flour mixture, mixing at low speed until smooth.
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8
Transfer batter to a large shallow bowl.
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9
Clean mixing bowl and beaters.
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10
Beat whites with a pinch of salt in mixer until they just hold stiff peaks, then fold into batter gently but thoroughly.
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11
Divide batter between pans, smoothing tops.
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12
Bake in middle of oven until cake is springy to the touch and a tester comes out clean, 15 to 20 minutes.
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13
(Layers will be about 3/4 to 1 inch high.)
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14
Cool in pans on racks 5 minutes.
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15
Run a sharp thin knife around edges, then invert onto racks and cool completely.
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16
Whisk together yolks, sugar, cornstarch, salt, and 2 tablespoons milk in a bowl.
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17
Bring remaining milk to a boil in a 2-quart heavy saucepan and add to yolk mixture in a slow stream, whisking constantly.
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18
Pour custard into saucepan and bring to a boil over moderately low heat, whisking constantly, then boil, whisking constantly, 2 minutes.
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19
Remove from heat and whisk in butter and vanilla.
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20
Transfer custard to a bowl and chill, its surface covered with wax paper, until cold, about 2 hours.
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21
Stir together evaporated milk, sugar, and corn syrup in a small heavy saucepan and simmer, stirring constantly, until caramel-colored and slightly thickened, about 6 minutes (be careful not to burn).
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22
Remove from heat and add chopped chocolate, whisking until smooth.
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23
Stir together cocoa, hot water, and vanilla in a small bowl, then stir into glaze.
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24
Chill, covered, until cool, about 15 minutes.
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25
Place 1 cake layer, upside down, on a platter.
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26
Whisk custard until smooth and spread 2/3 cup on top.
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27
Cut banana into 1/4-inch-thick slices and arrange evenly over custard, then spread remaining custard over banana.
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28
Top with remaining cake layer, upside down.
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29
Pour chocolate glaze over top (some may drip onto plate).
