Add butter, olive oil, garlic, and leeks to a large saucepan set over medium heat. Cook for 4-5 minutes, stirring occasionally, just until the leeks are tender. Do not allow to brown.

Add the potatoes and chicken broth. Increase heat to high and bring to a rapid boil. Immediately reduce heat to just below medium and cover pan loosely with a lid. Simmer for 25-30 minutes or until potatoes are fork-tender.

While the potatoes are simmering, combine the sour cream and heavy cream in a medium-sized bowl. Season with salt and black pepper, to taste, and stir to combine. Set aside.

Remove pan from heat and cool for several minutes. Working in batches, if necessary, carefully transfer the contents of the pan to a high-powered blender. Cover and blend on high for 1-2 minutes until completely smooth.

Remove lid and transfer the purée into a large bowl. Repeat the process with the remaining contents of the pan.

Temper the cream mixture by adding 2 or 3 tablespoons of the soup to the bowl. Stir to combine.

Add the tempered cream mixture to the pan with the puréed mixture. Season with additional salt and black pepper, as desired, and stir to combine.

Allow soup to cool completely, then transfer to an airtight container.

Refrigerate until chilled, approximately 3 hours to overnight.

To serve, divide the chilled soup between four bowls and garnish with fresh chives. Enjoy!