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1
Preheat the oven to 350F.
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2
Butter two 9-by-2-inch round cake pans; set aside.
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3
Into a large bowl, sift together flour and cornstarch; set aside.
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4
In the bowl of an electric mixer fitted with the whisk attachment, combine the whole eggs, egg yolks, granulated sugar, salt, and 1 teaspoon vanilla extract.
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5
Beat on high speed until thick and pale (it should hold a ribbon-like trail on the surface when the whisk is raised), about 5 minutes; scrape down the sides of the bowl as needed.
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6
Add flour mixture to egg mixture.
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7
With mixer on low speed, beat until just combined.
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8
Add oil in a steady stream, mixing until just combined.
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9
Remove bowl from mixer.
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10
Using the whisk, fold mixture several times.
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11
Divide batter between pans, and smooth with an offset spatula.
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12
Bake, rotating pans halfway through, until the cakes are springy to the touch and a cake tester inserted in the centers comes out clean, about 30 minutes.
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13
Immediately invert cakes onto a wire rack.
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14
Then reinvert cakes, and let them cool completely, top sides up.
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15
In the bowl of an electric mixer fitted with the whisk attachment, combine cream, confectioners sugar, and remaining teaspoon vanilla extract.
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16
Scrape in vanilla seeds, if using.
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17
Starting on low speed and gradually increasing to medium-high, whip until stiff peaks form, about 2 minutes.
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18
Using a serrated knife, trim the tops of the cakes to make level.
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19
Slice each cake in half horizontally into two layers.
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20
Place one of the bottom layers on a serving plate.
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21
Spread a quarter of the whipped cream over the layer; arrange a quarter of the mixed berries on top.
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22
Repeat with remaining cake layers, cream, and berries; garnish top with mint leaves, if using.
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23
Serve immediately; slice with a serrated knife.
