Instructions

• Preheat oven to 350°F (175°C). Lightly butter a 10-by-15-inch jelly roll pan, line the bottom with parchment paper, and lightly butter the parchment.
• Sift together 3/4 cup all-purpose flour, 1/4 cup dark/dutch-process cocoa powder, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon kosher salt into a bowl; set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, beat 4 large room-temperature eggs and 3/4 cup granulated sugar on medium-high until thick, pale, and ribboning, about 3 to 5 minutes. Mix in 1 teaspoon vanilla extract and 1/2 teaspoon instant espresso powder until combined.
• Place 1/4 cup unsalted butter (cut into cubes) and 2 ounces finely chopped dark chocolate in a microwave-safe bowl. Heat on medium power for 60 to 90 seconds, stirring every 30 seconds, stopping when the chocolate is almost melted so residual heat finishes melting it. (Alternatively, melt gently over a double boiler.) Let the mixture cool slightly.
• With the mixer running on low speed, drizzle the melted butter-chocolate mixture into the egg mixture until evenly incorporated. Scrape down the sides of the bowl as needed.
• Sprinkle the sifted dry ingredients over the batter and fold them in gently with a large rubber spatula until just combined and no streaks of flour remain. Be careful not to overmix.
• Spread the batter evenly into the prepared pan, using an offset spatula to get into the corners and create an even layer.
• Bake for 10 to 12 minutes, or until the cake springs back when lightly touched with a fingertip and a toothpick inserted in the center comes out mostly clean. Remove from the oven and let cool in the pan for 2 minutes.
• Place a piece of parchment paper on your work surface and lightly dust it with powdered sugar. Invert the warm cake out of the pan onto the sugared parchment, peel off the bottom parchment from the cake, and immediately (while warm) roll the cake up starting from a short side, using the parchment to help. Put the rolled cake seam-side down and let it cool completely, at least 30 minutes.
• While the cake cools, make the filling: in the bowl of the stand mixer with the paddle attachment, beat 6 tablespoons unsalted butter (room temperature) and 2 tablespoons vegetable shortening (room temperature) until fluffy, about 1 to 2 minutes.
• Add 8 ounces marshmallow fluff and beat until evenly incorporated. Add 1 cup sifted powdered sugar and 1/4 cup heavy cream; mix on low until incorporated, then increase to medium-high and beat 2 to 3 minutes until light and fluffy. If the filling is too loose, add more sifted powdered sugar a few tablespoons at a time (up to the optional amount indicated in the ingredient list) until it reaches a spreadable but stable consistency.
• Gently unroll the cooled cake, peeling back the parchment as you go. Spread an even layer of filling about 3/8-inch thick over the cake (you may not need all the filling). Starting from the same short side as before, roll the cake up tightly but gently so the filling does not squish out. Set seam-side down and refrigerate for at least 30 minutes to firm the filling.
• Make the glaze: place 4 ounces finely chopped dark chocolate in a heatproof bowl. Warm 3/4 cup heavy cream in a small saucepan over medium-low heat until it just begins to bubble around the edges (do not boil). Pour the hot cream over the chopped chocolate, let sit 30 seconds, then whisk from the center outward until smooth. Whisk in 1 tablespoon salted butter, one cube at a time, until melted and the glaze is glossy and smooth.
• Place the chilled cake roll on a wire rack set over a sheet pan. Pour the glaze evenly over the top and sides, using a spatula to cover any exposed areas, and let the glaze set at room temperature (or refrigerate briefly to speed setting).
• Slice the cake with a large chef’s knife using a single clean slicing motion (do not saw); wipe the knife clean between slices. Store the cake lightly covered in the refrigerator for up to 3 days. Let rest at room temperature about 30 minutes before serving.