Preheat oven to 375 degrees. Butter an 8-inch cake pan and line with parchment paper.
Melt chocolate and butter together in a medium bowl set over a saucepan of simmering water. Stir until smooth; let cool slightly.
Whisk sugar into chocolate mixture. Add eggs, one at a time, until thoroughly combined. Stir in vanilla, salt and cocoa powder. Pour batter into prepared pan and set aside.
In a small bowl, mash berries and jam together using a fork. Dollop berry mixture all over cake batter and gently swirl, using a figure eight motion. Bake for 20-25 minutes, until center is just set. Let cool.
Dust cake with powdered sugar and serve with whipped cream and fresh berries, if desired.
