How to Make It
Preheat the oven to 350°. In a standing mixer fitted with a paddle, beat the butter, sugars and honey at medium speed until smooth. Beat in the milk and vanilla. Add both flours, the cocoa, baking soda and salt and beat at low until combined. Flatten the dough into 2 disks, wrap in plastic and chill for 30 minutes.
Line 2 large baking sheets with parchment paper. Roll out each piece of dough between 2 sheets of plastic wrap to an 11-inch square. Trim to form 2 neat 10-inch squares. Transfer the squares to the baking sheets. Bake for about 20 minutes, shifting the pans, until the dough is slightly springy. Let the dough cool on the baking sheets for 5 minutes, then slide the parchment onto a work surface. While the squares are still warm, cut each one into 4 strips, about 2 1/2 inches wide. Trim the edges; you should have eight 9-by-2 1/2-inch wafers. Let cool.
In a small bowl, sprinkle the gelatin over the cold water and let stand until softened, 2 minutes. In a large metal bowl set over a pan of simmering water, whisk the egg yolks, confectioners' sugar, Marsala and salt until tripled in volume, about 4 minutes. Whisk in the gelatin until melted. Scrape the mixture into a standing mixer fitted with a whisk and beat at medium-high until cool, 5 minutes. Add the mascarpone and beat just until combined; transfer to another bowl.
Wipe out the mixer. Add the cream and whip until soft peaks form. Fold the cream into the Marsala-mascarpone filling.
Line a 9-by-5-inch loaf pan with enough plastic wrap to hang over on all sides. Tip the loaf pan on its side. Arrange the wafers on a work surface. Spread the filling thickly on all but 1 of the wafers. Stack the cream-topped wafers and top with the plain wafer. Slide the stack into the tilted loaf pan. Return the pan to its upright position so the wafers stand vertically. Fold the plastic tightly over the cake. Refrigerate for 24 hours, until the center wafers are softened.
In the bowl of a standing mixer fitted with a whisk, combine the egg whites and sugar. Set the bowl over a pan of simmering water and whisk until warm to the touch and the sugar is dissolved. Meanwhile, in a small bowl, sprinkle the gelatin over the cold water and let stand until softened, 2 to 3 minutes. Whisk the softened gelatin into the egg whites along with the espresso and beat at high speed until thick and fluffy, about 5 minutes.
Unwrap the cake and invert it onto a plate. Remove the plastic wrap. Using an offset spatula, spread the filling to fill any gaps. Spread about one-third of the meringue all over the cake and refrigerate until firm, 10 minutes. Spoon the remaining meringue into a large pastry bag fitted with a star tip. Pipe lengthwise stripes of meringue along the top and long sides of the cake, starting from the bottom and working up as you go. Refrigerate until firm, about 15 minutes.
Using a blow torch, toast the meringue until golden. Serve.