Melt the chocolate in a double boiler. Once melted, add the sugar and half-and-half, and stir until well blended. Stir in the butter and vanilla extract until the sauce is smooth and glossy.
Line up four small bowls or glasses, and place a dollop of the chocolate sauce in each. Evenly divide the blackberries, blueberries and raspberries between the four bowls. Top each with 2 tablespoons of the chocolate sauce, and serve.