1/4 cup (32 grams) all-purpose flour
1/4 cup (28 grams) cocoa powder
1/8 teaspoon salt
6 ounces (170 grams) bitter or semi-sweet chocolate, chopped
2 tablespoons (28 grams) unsalted butter
2 tablespoons (30 ml) water
6 eggs, separated, at room temparature
1/3 cup (67 grams) sugar or Swerve sweetener
1 teaspoon (5 ml) vanilla extract
1/8 teaspoon cream of tartar
3/4 cup blackberry jam
1 cup fresh blackberries, plus more for garnish
1 recipe whipped cream (substitute equal amount of Swerve for sugar free)
1 recipe chocolate ganache
To make the cake: Preheat oven to 400 degrees (200 C). Spray an 18×12-inch rimmed baking sheet with non-stick cooking spray with flour and line the bottom with parchment paper; set aside.
In a small bowl, combine the flour, cocoa powder, and salt; set aside.
in a medium bowl, combine the chocolate, water, and butter. Warm the mixture in the microwave for 15 seconds at a time, stirring between each time, until the chocolate is melted and smooth. Set aside to cool.
In the bowl of a standing mixer, beat the egg yolks at medium high speed until just combined (10 to 15 seconds).With the mixer still running, add about half of the sugar. Continue beating, scraping the bowl as needed, for 8 minutes. Stir int he vanilla, then transfer the yokk mixture to a large bowl.
Thoroughly wash and dry the mixer bowl and whisk.
In the clean bowl, beat the egg whites and cream of tartar at medium speed until foamy (about 30 seconds). Add 1 tablesppon of sugar and continue beating until soft peaks form, then gradually add the rest of the sugar. Beat until the whites are glossy and hold stiff peaks (about 1 minute). Do not overbeat.
Stir the chocolate mixture into the egg yolks. Once combined, add about 1/4 of the egg whites to the chocolate mixture, and gently fold it in with a rubber spatula. Fold in the remaining egg whites until combined, then sprinkle the dry ingredients over the top. Quickly and gently fold them in until combined.
Pour the batter into the prepared pan and bake for 8 to 10 minutes, rotating the pan half way through baking time.
While the cake is baking, lay a clean, dry dish towel on a flat surface. Sprinkle about 1 tablespoon of cocoa powder over it and rub it into the towel with your hands.
Once the cake is done, overturn it onto the prepared towel, peel away the parchment, and immediately roll the short side of the cake up in the towel. Transfer the roll to a wire cooling rack and allow to cool for at least 15 minutes.
Assemble the cake: Once cooled, unroll the cake and spread the jam over the surface, leaving a 1-inch border around the edge. Next, spread about 1 cup of whipped cream over the top of the jam. Sprinkle with the blackberries, and re-roll the cake. Trim both ends using a serrated knife. Drizzle with ganache and garnish with remaining blackberries.