Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9 inch round cake pan.
In a medium bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa powder, baking powder and salt; stir into the batter just until blended.
Spread the batter into the prepared pan. Bake for 25 to 30 minutes in the preheated oven, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool for 10 minutes, then turn out onto a wire rack to cool completely.
In a medium bowl, beat the cream until stiff peaks form. Fold in the melted chocolate until blended. Spread the chocolate mousse over the cooled cake.
Spread the strawberry jam over the mousse. Refrigerate the cake for at least 1 hour before serving.