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1
Put oven rack in middle position and preheat oven to 350F.
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2
Butter cake pan.
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3
Line bottom of pan with a round of wax paper or parchment, then butter paper.
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4
Dust pan with flour, knocking out excess.
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5
Sift together flour and salt.
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6
Beat together eggs, sugar, and vanilla with an electric mixer at high speed until tripled in volume and thick enough to form a ribbon that takes 2 seconds to dissolve when beater is lifted, 7 to 8 minutes in a standing mixer or 14 to 16 minutes with a handheld.
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7
Resift flour mixture, one third at a time, over batter, folding in gently but thoroughly with a rubber spatula after each addition.
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8
Stir together butter and about 3/4 cup batter in a small bowl until combined, then fold butter mixture into batter gently but thoroughly.
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9
Pour batter into cake pan and bake until a wooden pick or skewer inserted in center comes out clean, 30 to 40 minutes.
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10
Transfer cake in pan to a rack, then run a thin knife between cake and side of pan and cool cake, about 20 minutes.
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11
Invert cake onto a platter with a lip and peel off paper.
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12
Bring water, sugar, and salt to a boil in a 2-quart heavy saucepan, stirring until sugar is dissolved.
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13
Reduce heat and simmer, uncovered, 2 minutes.
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14
Add kumquats and simmer, uncovered, stirring occasionally, until tender, 10 to 12 minutes.
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15
Transfer kumquats to a heatproof bowl using a slotted spoon, then boil syrup until reduced to about 2/3 cup, 3 to 7 minutes.
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16
Pour over kumquats.
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17
Bring Sauternes, sugar, and zest to a boil in a 1-quart heavy saucepan, stirring until sugar is dissolved, then boil until reduced to about 1 cup, 5 to 8 minutes.
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18
Discard zest.
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19
Prick top of cake all over with wooden pick or skewer, then brush or pour Sauternes syrup, little by little, evenly over cake, letting syrup be absorbed before each new addition.
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20
Mound kumquats on cake using slotted spoon, then sprinkle with almonds.
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21
Serve remaining kumquat syrup on the side.
