Make the Berry 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.

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, baking 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. The mixture will be thick. Add the sour cream and whisk until well combined. The mixture will thin out. Add the melted butter and vanilla extract and whisk together until well combined.
4. Add the dry ingredients and gently whisk together until well combined, but don’t over mix. Mixture might be a little lumpy. Set aside.
5. Add the egg whites to a large mixer bowl and whip on high speed until stiff peaks form.
6. Gently fold about 1/3 of the whipped egg whites into the batter to loosen up the batter. Add the remaining egg whites and gently fold together until well combined.
7. 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.
8. 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.
9. 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.
10. 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 it’s 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 another 15 minutes or so. It should take about an hour to an hour and a half to cool completely, depending on the temperature of your home.

Make the Whipped Cream Filling and Assemble

1. When the cake has cooled and is ready to be filled, make the whipped cream filling. Add the heavy whipping cream, powdered sugar, vanilla extract and salt to a large mixer bowl and whip on high speed until the cream begins to thicken.
2. Add the mascarpone cheese to the whipped cream and whip on low speed until stiff peaks form. It will happen fairly quickly.
3. 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.
4. Spread the berry 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 sides.
5. To add the whipped cream filling, I like to pipe it evenly on top of the berry filling because just spreading it can cause the berry 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 berry filling and then gently spread, as needed, to create a full and even layer.
6. 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.
7. 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.
8. When you’re read to decorate and serve the cake, remove it from the plastic wrap and place it on a serving platter. Dust the top with additional powdered sugar, then decorate as desired with whipped cream and fresh berries. I piped 5 large swirls of whipped cream on top and added the berries on top.
9. Cover and refrigerate until ready to serve. Best if eaten within 3-4 days.