Make the filling

1. Add the sugar and cornstarch to a medium-sized saucepan and whisk together until no lumps remain. Add the water and lemon juice and stir together.
2. Heat over medium heat until the sugar begins to melt, then add the berries. Stir to coat with the sugar mixture. Continue to cook until the berries start to soften and let out juices. Use your spatula to mash the berries a bit.
3. Continue to cook and then remove from heat when the juices have begun to thicken, after about 3-4 minutes. Pour the mixture into another bowl and refrigerate until cool. Mixture will thicken as it cools. If you’d like to remove the seeds, you can strain the mixture right after you finish cooking it. You can make the filling up to 2-3 days ahead, if you’d like. It should be completely cooled before adding it to your cake roll.

Make the cake roll

1. Preheat oven to 350°F. Line a 17×12-inch jelly roll sheet pan with parchment paper. Make sure the parchment paper sticks up at least an inch above the sides of the pan on all sides. You’ll use the parchment paper later to lift the cake out of the pan and roll it up.
2. Whisk the flour, cocoa, baking powder, espresso powder and salt together in a medium bowl and set aside.
3. In a large bowl, combine the egg yolks and sugar and whisk together until well combined.
4. Add the sour cream, melted butter and vanilla extract and whisk together until well combined.
5. Add the dry ingredients and gently whisk together until well combined, then set aside.
6. Add the egg whites to a large mixer bowl and whip on high speed until stiff peaks form.
7. Gently fold about 1/3 of the whipped egg whites into the chocolate mixture to loosen up the batter.
8. Add the remaining egg whites and gently fold together until well combined.
9. Spread the cake batter evenly into the prepared pan and bake for 10-12 minutes, or until the top of the cake springs back when touched and a toothpick inserted comes out clean.
10. Remove the cake from the oven and immediately lift the cake out of the pan using the parchment paper and place it on the counter.
11. While the cake is hot, use the parchment paper the cake was baked in and start at the shorter end of the cake to slowly roll the cake up. You can just set the cake roll aside to cool completely, if you like. But to help avoid having the cake crack later, I like to unroll and re-roll the cake a few times during cooling to help it keep its elasticity. I first unroll it after about 5 minutes, then after another 5 minutes, and again after another 10 minutes. From there, I might roll it out 2-3 more times after 10-15 minute intervals. It should take about an hour to an hour and a half to cool completely, depending on the temperature of your home.

Make the filling

1. When the cake has cooled and is ready to be filled, make the whipped cream filling. Add the heavy whipping cream, powdered sugar, cocoa powder and vanilla extract to a large mixer bowl and whip on medium speed until it just reaches stiff peaks.
2. Unroll the cake roll very carefully, looking out for areas where it may be sticking to the parchment paper to release it. You can use an offset spatula or something similar and run it along the parchment paper as you unroll the cake to help release it as it unrolls. I also like to gently lift up before I roll back, to give the cake a chance to release.
3. Spread the raspberry filling evenly onto the unrolled cake, leaving some space around the edges for it to move around a bit as you re-roll it. I recommend about ½ an inch on the long sides and an inch or two on the short side.
4. To add the chocolate whipped cream filling, I like to pipe it evenly on top of the raspberry filling because just spreading it can cause the raspberry filling to move around and become an uneven layer. If you don’t have any piping bags, you can use a Ziplock bag and snip a corner off and use that as a “piping bag”. Pipe the whipped cream on top of the raspberry filling and then gently spread, as needed, to create a full and even layer.
5. Roll the cake roll back up without the parchment paper. After you roll it up, some of the filling and whipped cream might be squeezed out of the side, depending on how much room you left on the edges when adding the filling. Just use a spatula or something similar to remove it, if needed.
6. Wrap the cake roll up in plastic wrap with the seam side down and refrigerate for at least an hour to firm up. If you are prepping it ahead, this is when I would refrigerate it overnight. You could also go ahead and prep the chocolate ganache in the next step.

Prepare chocolate ganache and finish it off

1. To make the chocolate ganache, add the chocolate to a medium sized bowl and set aside. Heat the cream in the microwave just until it begins to boil, then pour it over the chocolate.
2. Allow the chocolate and cream to sit for a few minutes, then stir until smooth.
3. Chill the chocolate ganache until cold and firm. When you’re ready to use it to decorate your cake, heat in the microwave for 10-15 seconds at a time, stirring between each interval, until it’s spreadable. You don’t want it fully melted. It should be thick enough that it’ll stay in place on the cake. You just want it warm enough to be able to spread it easily. I heated mine for just 30 seconds total.
4. Remove the chilled cake roll from the fridge and plastic wrap and place on a serving plate or platter. Spread the chocolate ganache onto the cake roll. Decorate with additional chocolate whipped cream and raspberries, if desired.
5. You can serve the cake roll immediately, or refrigerate until the chocolate ganache is cold and firm, 1-2 hours. Slice the cake roll and serve.
6. Cake roll is best stored well covered and eaten within 3-4 days.