Preheat oven to 180 degrees C (350 degrees F).
Beat Salted Butter (125 gram) for 10 minutes on high speed until pale and fluffy. Set aside.
Beat Egg (10) and Caster Sugar (125 gram) in another bowl for 10 minutes on high speed.
Lower your mixer speed and stir in All-Purpose Flour (50 gram) and Milk Powder (2 tablespoon). Then, drizzle in Vanilla Extract (1 teaspoon), and add in the butter.
In a small bowl, mix Unsweetened Cocoa Powder (1 tablespoon) and a tablespoon of water until well combined.
Separate the egg mixture into 3.
Stir in cocoa powder mixture with one of the bowl.
Transfer each mixture into a loaf (7.9" x 4.3") lined with parchment paper, and bake in the oven for about 20 minutes.
Spread with some Strawberry Jam (2 tablespoon) and serve!