1. Preheat the oven to 350°F.
2. Separate the egg whites from the yolks.
3. Melt the chocolate and set aside.
4. With the mixer, beat the sugar with the egg yolks for 2 minutes. Add the melted butter. Add the flour, baking powder, and salt. Mix slowly.
5. Beat the egg whites until stiff peaks form and gently fold into the batter.
6. Pour half of the mixture into a greased and floured mold.
7. Place the blackberry jam in the center and cover with the remaining batter.
8. Bake for 25 minutes.
9. Remove from the oven and let cool on a rack. Unmold.
10. Decorate with fresh blackberries before serving.