Cream butter, sugar and vanilla in a bowl. Add eggs, beat until blended. Sift together flour, baking powder and salt. Stir into creamed mixture. Place half of the batter in a seperate bowl. Blend 1 ounce of melted chocolate into one of the bowls. Place the batter with the chocolate added into an 8x8x2 inch baking pan. Pour the reserved bowl of batter over the top slowly while swirling with a knife to marble the brownies. Bake at 350 degrees for 30 minutes.