Add butter, olive oil, garlic, and leeks to a large, high-sided skillet set over medium heat. Cook, stirring occasionally, just until the leeks are tender, approximately 4-5 minutes. Do not allow to brown,
Add potatoes and chicken broth. Increase heat to high and bring to a rapid boil. Immediately reduce heat to just below medium and cover skillet 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 skillet from heat and cool for several minutes. Working in batches, if necessary, carefully transfer the contents of the skillet to a high-powered blender.Cover and blend on high until completely smooth, approximately 1-2 minutes.