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1
Preheat the oven to 350 degrees F.
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2
Center a rack in the oven.
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3
Butter a 9-inch round cake pan.
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4
Dust with flour, tapping out the excess.
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5
Pour a few inches of water into a skillet large enough to hold the bowl from your mixer.
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6
Bring the water to a gentle simmer.
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7
Melt the butter and set it aside to cool.
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8
It should be just warm when you're ready for it.
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9
Whisk the eggs and sugar together in the mixer bowl.
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10
Place the bowl in the skillet of simmering water and, whisking without stop, heat the mixture until it is foamy, slightly pale, and reads between 130 and 140 degrees F on an instant-read thermometer, about 4 minutes.
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11
Remove the bowl from the water.
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12
Working in the mixer fitted with the whisk attachment, beat the mixture on high speed until it cools to room temperature and triples in volume.
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13
When you lift the whisk, the batter will fall back into the bowl and form a ribbon that remains on the surface for 10 seconds before it dissolves, about 5 to 8 minutes.
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14
Stir about 2 tablespoons of batter into the slightly cooled butter and set it aside.
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15
With a large flexible rubber spatula, gently fold the flour into the batter in 2 or 3 additions, taking care to handle the batter gently in order to maintain its bubble structure.
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16
(The batter will lose volume as you fold in the flour and later the butter.
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17
This reaction is inevitable and shouldn't jeopardize the success of the finished cake, but be careful just the same.)
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18
Still working with the spatula, fold in the butter mixture.
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19
Pour the batter into the prepared pan and bake until the top is golden and springy to the touch and a toothpick inserted in the center of the cakes comes out clean, 28 to 33 minutes.
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20
Transfer the cake to a cooling rack for 5 minutes.
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21
Remove cake from pan, invert, and cool completely on the rack.
