This recipe features a silky whipped-cream filling that's whisked with caramel, then rolled up in a genoise cake that's been brushed with bourbon-chocolate syrup for unbeatable flavor that's nuanced and sophisticated.
Whipped cream: Fill a large bowl with ice water. In a small, heavy pot, stir together granulated sugar and 2 tablespoons water; heat over medium-high, stirring occasionally, until sugar is dissolved. Continue to cook, swirling occasionally, until deep golden brown, about 6 minutes. Meanwhile, stir cornstarch into 1/4 cup cream, then stir mixture into remaining 1 1/4 cups cream. Add cream mixture and salt to caramel, whisking constantly (it will bubble up), and bring to a boil. Set pot in bowl of ice water and let cool, whisking occasionally, 30 minutes; then refrigerate until very cold, at least 1 hour and up to overnight.
Cake: Preheat oven to 425 degrees. Spray a 12 1/2-by-17 1/2-inch rimmed baking sheet with cooking spray. Line with parchment; spray parchment, then dust with cocoa, tapping out excess. Whisk together cocoa, flour, and salt.
Combine eggs, yolks, and granulated sugar in a heatproof bowl set over (not in) a pot of simmering water. Whisk until sugar is dissolved and mixture is warm to the touch, about 2 minutes. In a mixer fitted with the whisk attachment, beat on medium-high speed 2 minutes. Increase speed to high and beat until mixture is pale and thick, about 4 minutes more.
Sift cocoa mixture over top of egg mixture, then fold in. When almost incorporated, pour butter down side of bowl; gently fold to combine, scraping bottom of bowl. Transfer to prepared sheet, using an offset spatula to spread evenly. Bake until center springs back when lightly touched, 6 to 7 minutes. Run a small sharp knife around edges of cake. Invert onto a clean kitchen towel; remove parchment. Immediately, starting at one short side, roll into a log, incorporating towel. Let cool 30 minutes.
Syrup: In a small, heavy pot, stir together granulated sugar and 2 tablespoons water; heat over medium-high, stirring occasionally, until sugar is dissolved, 1 minute. Continue to cook, without stirring but swirling occasionally, until syrup turns dark amber, 3 to 4 minutes. Whisk in 1/4 cup water (it will bubble up), bourbon, and cocoa. Whisk until smooth. Let cool about 30 minutes.
Whisk confectioners' sugar into chilled cream mixture, then whip to stiff peaks. Refrigerate at least 30 minutes and up to 2 hours. Whisk again just before filling cake.
Unroll cooled cake. Brush surface generously with syrup. Use offset spatula to spread whipped cream over cake, leaving a 1/2-inch border all around. Starting at a short end, roll cake to enclose filling (without towel). Wrap towel around cake (for a tight roll, hold closed with binder clips). Refrigerate at least 1 hour and up to overnight. Dust with cocoa and serve. (For cleanest slices, use a serrated knife.)