Grease a 9x5x3-inch loaf pan or an 8x1-1/2-inch round baking pan. For batter, in a medium mixing bowl beat the margarine or butter with an electric mixer on medium to high speed about 30 seconds or until softened.
Add about half of the flour to the margarine or butter. Then add the sugar, buttermilk or sour milk, egg white, baking powder, baking soda, and vanilla. Beat on low speed until thoroughly combined, scraping the sides of the bowl. Beat on medium speed for 2 minutes more. Then beat in the remaining flour on low speed just until combined.
Spread the batter into the prepared pan. Dollop jam in small spoonfuls on top of the batter. Using a small narrow spatula or knife, gently swirl preserves into the batter to create a marbled effect. Sprinkle the pecans or almonds over the top.
Bake in a 350 degree F oven for 25 to 30 minutes or until a wooden toothpick inserted near the center comes out clean. Remove from oven. Sprinkle chocolate or vanilla-flavored pieces over warm cake. Let stand 5 minutes. Use the tines of a fork to swirl the pieces. Let cake cool an additional 10 minutes before serving. Serve warm or cool. Makes 4 servings.