Preheat oven to 350ºF. Lightly butter a 10-by-15-inch jelly roll pan; line bottom with parchment paper and lightly butter parchment.Sift together flour, cocoa powder, baking powder, baking soda, and salt and set aside.In a stand mixer fitted with the paddle attachment, beat eggs and sugar until thick and pale, 3 to 5 minutes. Mix in vanilla and espresso powder.Gently melt butter and chopped chocolate in the microwave for 60 to 90 seconds on medium power, stirring every 30 seconds. Stop heating when the chocolate is almost melted, the residual heat should be enough to melt it the rest of the way.With the mixer running on low speed, drizzle in the melted chocolate mixture until evenly incorporated, scraping down the sides of the bowl as necessary.Sprinkle dry ingredients over top, and fold into batter using a large rubber spatula. Spread into prepared pan, using an offset spatula to spread the batter evenly into the corners.Bake for 10 to 12 minutes or until the cake springs back when touched with a fingertip. Remove from oven and let cool for 2 minutes, then turn out the cake upside down onto a piece of parchment paper sprinkled lightly with powdered sugar. You can also use a clean dish towel. Peel away parchment paper from bottom of cake, then gently roll up cake starting with the short side. Doing this while the cake is still warm will give the cake some ‘muscle memory’ and allow it to be rolled with the filling without cracking. Set the rolled-up cake seam-side down, and let cool completely, at least 30 minutes.While cake is cooling, prepare the filling. In the bowl of your stand mixer fitted with the paddle attachment, beat butter and shortening together until fluffy. Add marshmallow fluff and beat until evenly incorporated, then add sifted powdered sugar, beating on medium speed until fluffy, scraping down the sides of the bowl as needed. Add heavy cream and mix until incorporated, then increase speed to medium-high and beat for 2 to 3 minutes or until light and fluffy. If the filling is still rather loose, you can add 1/4 to 1/2 cup more powdered sugar as needed to achieve desired consistency.Gently unroll cooled cake, peeling back parchment paper as you go. Spread a 3/8-inch thick layer of filling over the top of the cake. Don’t make this layer too thick or you’ll find yourself with a big old mess when you try to roll it up (you may not need all of the filling). Starting with the same short side as before, carefully roll up the cake, trying not to let the filling squish out too much. Set seam side down, then refrigerate for at least 30 minutes to allow the filling to set up a bit (this will make it much easier to slice later).Meanwhile, to make the glaze, place chopped chocolate in a heat-proof bowl. Warm cream in a small saucepan over medium-low heat until it just begins to bubble around the edges (do not let it boil). Pour evenly over chopped chocolate; let sit for 30 seconds, then gently whisk, in concentric circles starting in the very center, until chocolate is melted and mixture is smooth. Whisk in butter, one cube at a time, until melted and completely smooth.Transfer cake roll to a wire rack set inside a sheet pan (this will catch any excess glaze). Pour glaze over top, spreading as needed to get any remaining exposed areas, until top and sides of cake are completely covered in glaze.Slice cake with a large chef’s knife, using a single clean slicing motion (do not saw the knife back and forth). Wipe knife clean between slices.Cake is best enjoyed the day it is made, but will also keep in the refrigerator, lightly covered with plastic wrap, for up to 3 days. Let sit room temperature for 30 minutes before serving.