Beat the butter, sugar and salt until fluffy. Add the eggs and stir until creamy. Mix the flour and baking powder in a separate bowl. Alternately add the flour mixture and milk to the egg mixture, stirring constantly. If the batter is too hard, add 2-3 tablespoons of milk.
Grease a loaf pan with butter. Preheat the oven to 180°C (approximately 350°F). Fill 2/3 of batter into the loaf pan. Mix the remaining 1/3 of the batter with the cocoa and 3 tablespoons of milk.
Pour the dark batter onto the light batter and mix with a fork, forming a marbled pattern. Bake for 60-70 minutes, until golden brown. Remove from the oven and let cool. Sprinkle with powdered sugar and serve.