01 - Melt the unsalted butter in a large saucepan over medium heat. Add leeks and onion; sauté for 5 to 7 minutes until softened without browning.
02 - Incorporate the diced potatoes and cook for 2 minutes, stirring occasionally for even heating.
03 - Pour in the vegetable broth. Raise the heat until gently boiling, then lower to a simmer, cover, and cook for 20 minutes until potatoes are tender.
04 - Remove saucepan from heat and allow the soup to cool slightly. Purée with an immersion blender or in batches using a countertop blender until completely smooth.
05 - Stir in heavy cream, salt, and white pepper. Taste and adjust seasoning if necessary.
06 - Transfer the mixture to a bowl, cover, and refrigerate for a minimum of 2 hours until the soup is well chilled.
07 - Ladle chilled soup into serving bowls and garnish with fresh chives before serving.