Preheat your oven to 350. Find a 12 x 17 (or similar) baking sheet with sides, also called a jelly roll sheet pan. Line it with parchment paper and spray with a flour based baking spray as well.
Dust a tea towel with powdered sugar and set aside to roll your cake in later.
Sift coco, flour, baking powder and salt together in a bowl and set aside.
Separate your eggs. Melt your butter and set aside to let it cool off before adding.
In a standing mixer, fitted with whisk attachment, risk yolks and sugar until combined. Add melted butter, sour cream and vanilla.
Add dry ingredients to mixture. Remove bowl from mixer and set aside.
In a clean bowl, with a clean whisk attachment, whip egg whites on high speed until stiff peaks form. Gently fold whites into the rest of the batter. You want to be careful to keep as much air in your mixture as possible. Don’t over mix.
Then spoon batter on to sheet pan and spread evenly. Bake at 350 for 10-13 min.
Once baked, immediately turn cake onto sugared tea towel. Peel off old parchment and add a new sheet to the top. Now quickly roll the cake up with the tea towel, from the shorter end. Let cool in towel for at least 1 hour.
While it cools lets work on fillings.
In a pot over medium heat, combine milk, pistachios and salt, and heat until just simmering. Remove from heat and allow to sit for 15 minutes. Strain through a fine sieve and discard solids. Measure out 1½ cups of the pistachio milk. Add more milk if needed.
In a separate bowl, whisk together sugar and egg yolks until glossy and well combined. Whisk in cornstarch. Slowly add pistachio milk to the egg mixture, whisking constantly until cream thickens and ingredients are evenly incorporated.
Strain mixture into a saucepan and cook over medium heat, stirring constantly, until a slow boil begins. Allow mixture to boil 1-2 minutes or until well thickened, stirring constantly. Remove from heat and stir in butter and vanilla. Pass mixture through a fine sieve and transfer to a mixing bowl. Refrigerate until completely cooled.
Whip cream until hard peaks form. Fold into pastry cream. Reserve refrigerated. When you are ready to assemble your cake, you will want to fold the cream and pistachio mixture together.
Once your cake has cooled for at least an hour, and your filling has cooled for two you can begin assembly.
Gently unroll your cake. Line the cake with a thin layer of sliced strawberries.
Cover with a layer of pistachio cream, spooning it on and then spreading helps. Now, add a layer of broken raspberries to the cream. You can add pistachio crumbs too if you like.
Prepare a large piece of plastic wrap and have it ready on the counter because now it is time to carefully re-roll your cake. Roll the cake back up and transfer to the plastic. Be gentle so you don’t squish all your fillings right out the sides. Wrap tightly in plastic wrap and refrigerate, seam side down, for at least three hours. Even better if you leave it over night.