How to Make It
In a large bowl, combine the milk chocolate and instant espresso. In a saucepan, heat the cream. Pour the cream over the chocolate; let stand until melted, then whisk until blended. Refrigerate the mocha cream until cold, about 2 hours.
Preheat the oven to 350°. Spray a 12-by-16-inch jelly roll pan with vegetable oil spray. Line the bottom with parchment paper and spray the paper. Spread the pecans on a pie plate and bake for 7 minutes, or until golden. Let cool completely. In a food processor, pulse the pecans with the flour until finely ground.
Put the bittersweet chocolate and butter in a heatproof bowl; set the bowl over a saucepan of barely simmering water and stir until melted. Off the heat, whisk in the yolks, 1/3 cup of the sugar and the salt.
In a large bowl, beat the egg whites and cream of tartar until frothy. Beat at high speed until soft peaks form. Slowly beat in the remaining 1/3 cup sugar until the whites are firm and glossy. Fold one-fourth of the whites into the chocolate; fold the chocolate and the pecans into the remaining whites until no streaks remain.
Spread the batter in the prepared pan and bake for 9 minutes, or until springy. Let the cake cool in the pan on a rack.
Run a knife around the edge of the pan. Heavily dust the cake with cocoa powder and cover with a sheet of foil. Place a cutting board over the cake and invert it; remove the pan and parchment paper.
Beat the chilled mocha cream at high speed until it is firm and holds its shape. Spread the cream evenly over the cake. Starting at a long side, roll up the cake as tightly as possible, using the foil as a guide. Tightly wrap the cake roll in the foil and slide it onto a cutting board. Refrigerate until firm, about 2 hours. Unwrap the cake and dust with more cocoa. Carefully slide it onto a long platter and dust with confectioners' sugar. Cut into slices; serve.