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1
Preheat the oven to 350F.
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2
Line a 17-by-12-inch rimmed baking sheet with parchment paper.
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3
Dust a large, clean kitchen towel with confectioners sugar; set aside.
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4
Into a medium bowl, sift together the flour, baking powder, and salt; set aside.
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5
Place the whole egg, egg yolks, vegetable oil, honey, 1/4 cup granulated sugar, vanilla, and warm water in the bowl of an electric mixer fitted with the whisk attachment.
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6
Beat on medium speed until thick and pale (it should hold a ribbon-like trail on the surface when the whisk is raised), 6 to 7 minutes.
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7
Transfer to a large bowl; set aside.
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8
With a clean bowl and whisk attachment, beat the egg whites on medium speed until soft peaks form, 1 to 2 minutes.
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9
Raise speed to medium-high; gradually add remaining 3 tablespoons granulated sugar, beating until stiff peaks form, 2 to 3 minutes.
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10
Using a rubber spatula, fold a third of the egg-white mixture into the egg-yolk mixture to lighten.
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11
Gently fold in remaining whites until just combined.
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12
Sift flour mixture over the top, and gently fold in until just combined.
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13
Pour batter onto prepared baking sheet; using an offset spatula, spread evenly.
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14
Bake, rotating sheet halfway through, until the cake is golden and springs back when lightly touched in the center, about 10 minutes.
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15
Immediately invert cake onto prepared towel; carefully lift off pan and peel off parchment paper.
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16
Starting on a long side, roll towel and cake into a log, incorporating the towel as you go.
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17
Transfer cake (still in towel), seam side down, to a wire rack.
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18
Let cool completely.
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19
In a medium bowl, crush the blackberries with a fork; set aside.
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20
In the bowl of an electric mixer fitted with the whisk attachment, beat cream with the confectioners sugar on medium-high speed until stiff peaks form, about 2 minutes.
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21
Fold in crushed blackberries; set aside.
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22
Unroll cake, leaving it on the kitchen towel.
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23
Using an offset spatula, spread blackberry mixture over top of cake, leaving a 1-inch border on both long sides.
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24
Gently roll cake again into a log (do not incorporate towel).
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25
Wrap with towel; refrigerate on a baking sheet until ready to serve, for at least 1 hour or up to 1 day.
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26
Remove towel, and generously dust cake with confectioners sugar before serving; slice with a serrated knife.
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27
While its still warm, a sheet of freshly baked sponge cake is rolled in a clean kitchen towel that has been dusted with confectioners sugar.
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28
Once it has cooled, the cake is unrolled and covered with a thick layer of blackberry fool before it is rolled and dusted once again.
