Set your oven to 350°F. Line a 9x5-inch loaf pan with parchment paper, leaving some overhanging on the sides for easy lifting. Lightly butter the parchment.

In a medium bowl, beat together ¼ cup softened butter and ½ cup sugar with a hand mixer until light and fluffy, about 2 minutes. Add two eggs, one at a time, mixing well after each.

Stir in the sour cream, vanilla extract, ground pistachios, and pistachio paste until smooth. Sift in one cup of flour, one teaspoon of baking powder, and ¼ teaspoon of salt. Gently fold the mixture with a spatula until combined. Do not overmix.

In another bowl, beat the remaining ¼ cup butter with ½ cup sugar. Add one egg, then the sour cream. Sift in one cup of flour, Dutch-process cocoa powder, one teaspoon of baking powder, and ¼ teaspoon of salt.

Pour in the milk and fold until just combined. This batter will be slightly thicker than the pistachio batter.

Put half of the pistachio batter into the loaf pan and spread it evenly. Add the chocolate batter on top and spread it gently. Then, add the remaining pistachio batter as the final layer.

Use a thin skewer or butter knife to make two to three slow S-shapes in the batter from one end of the pan to the other. Do not over-swirl; two to three passes will help you maintain the distinct marble look.

Bake at 350°F for 55 to 65 minutes. Start checking at 55 minutes by inserting a toothpick into the center. It should come out clean or with a few moist crumbs. The top may crack slightly, which is normal.

Let the loaf cool in the pan for 15 minutes. Use the parchment paper to lift the loaf out, then cool it completely on a wire rack. Drizzle melted white chocolate on top and sprinkle chopped pistachios for decoration.