Preheat the oven to 350°F (175°C) and grease a loaf pan.
In a large bowl, cream together the butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually alternate adding the sifted flour and milk to the mixture, mixing until well combined.
Divide the batter into three equal parts. Stir cocoa powder into one part, strawberry jam into another part, and leave the last part plain.
Spoon each batter into the prepared loaf pan alternating the flavors, then use a knife to swirl them together gently to create a marbled effect.
Bake for 50-55 minutes or until a toothpick inserted into the center comes out clean.
Allow the loaf to cool in the pan for about 10 minutes before removing it to a wire rack to cool completely.