While most cake rolls seem time consuming and daunting, this version is foolproof, thanks to an easy and fun method of assembling the cake layer. We added delicious strawberries and dark chocolate for the ultimate dessert.
Preheat the oven to 350 degrees F. Coat a 9-by-13-inch baking pan with nonstick cooking spray.
Prepare the cake mix as directed; pour the batter into the prepared pan. Bake until a cake tester inserted in the center of the cake comes out clean, about 30 minutes. Let cool completely.
Meanwhile, combine the butter and cream cheese in the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on medium-high speed until combined and fluffy, about 2 minutes. Reduce the speed to low and gradually add in the confectioners' sugar. Increase the speed to medium-high and beat until fluffy and combined, about 2 minutes.
Crumble the cake into a large bowl. Add 1/4 cup of the frosting and mix with your hands until smooth and combined.
Line a 15 1/2-by-10 1/2-inch jelly roll pan with plastic wrap. Put the cake "dough" on top. Cover with another sheet of plastic wrap and pat the mixture into an even rectangle that fills the pan. Transfer the mixture in the plastic to a work surface (the shorter side facing you) and roll out just enough so that the surface is smooth. This helps make the dough even and not patchy. Press the flat side of a ruler against all the sides of the dough so they are straight. The dough should be about 10-by-15 inches.
Put the freeze-dried strawberries in a spice grinder and pulse until finely ground. Add to the bowl with the remaining frosting and beat until combined.
Remove the top sheet of plastic wrap from the dough. Add the strawberry frosting and spread out evenly with a small offset spatula, leaving about a 1 1/2 inch border on the short side furthest away from you.
Roll the dough away from you into a jelly roll shape, using the plastic to roll tightly until it's covered by the plastic. Place it on a baking sheet. Freeze while you make the chocolate coating.
Combine the dark chocolate and coconut oil in a heatproof bowl set over a pan of simmering water. Stir until melted and smooth, about 6 minutes. Remove the bowl from the saucepan and set aside until cooled and thickened slightly, about 7 minutes.
Meanwhile, unwrap the jelly roll cake and place on a rack set over a baking sheet.
Pour the chocolate over the cake until completely covered. Decorate the top with freeze-dried strawberries. Freeze until the chocolate is set, about 5 minutes. Transfer to a serving platter and cut into slices. Serve chilled.