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1
Preheat the oven to 325F.
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2
Line a 17-by-12-inch rimmed baking sheet with parchment paper; set aside.
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3
Dust a large, clean kitchen towel with cocoa; set aside.
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4
Into a medium bowl, sift together the cocoa and flour.
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5
In a small bowl, stir together the espresso powder and the hot water until dissolved; set aside.
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6
In the bowl of an electric mixer fitted with the whisk attachment, beat the egg yolks on high speed until thick and pale (they should hold a ribbon-like trail on the surface when the whisk is raised), about 5 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, whip the egg whites on low speed until foamy.
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9
Raise speed to medium-high, and add the sugar in a slow, steady stream, beating until stiff peaks form.
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10
With 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 egg whites until just combined.
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12
Fold in the espresso mixture, then gradually fold in the cocoa mixture until just combined.
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13
Do not overmix.
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14
Pour batter onto prepared sheet; using an offset spatula, spread evenly and smooth the top.
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15
Bake, rotating pan halfway through, until the cake springs back when lightly touched, 10 to 12 minutes.
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16
Immediately invert cake onto prepared towel; carefully lift off pan and peel off parchment paper.
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17
Starting on a long side, roll towel and cake into a log, incorporating the towel as you go.
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18
Transfer cake (still in towel), seam side down, to a wire rack to cool completely.
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19
Unroll cake, leaving it on the kitchen towel.
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20
Using an offset spatula, spread Mocha Mousse evenly over the cake.
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21
Gently roll cake again into a log (do not incorporate towel).
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22
Wrap with the towel; refrigerate on a baking sheet until filling is well chilled, for at least 2 hours or up to 1 day.
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23
When ready to serve, generously dust roulade with cocoa; slice with a serrated knife.
