Adapted from my friend Jackie's recipe. Slurp it straight out of an ice-cold mason jar on a hot day, or get fancy and serve it amuse bouche-style in an appetizer spoon with a rose of pastrami-smoked salmon, a sprinkle of crushed dill pickle potato chips, and a smattering of snipped chives. —Chris Hagan
Halve the leeks lengthwise and cut crosswise into 1/4-inch strips. Wash well and dry.
Slowly render bacon in a soup pot or large saucepan over low heat. When fat has rendered, add the butter to the pan. When it stops foaming, add the leeks, garlic, a generous pinch of salt, and lots of fresh-ground black pepper, then cover and let stew until quite soft, 15-20 minutes.
Add the potatoes and stir to coat them in the fat and aromatics. Add broth and bring to a boil, then cover, reduce heat, and simmer until the potatoes are tender enough that they can be mashed against the side of the pot with your spoon.
Remove the pot from heat. Allow to cool a bit. Puree the soup in batches in a blender or food processor, or directly in the pan with an immersion blender. (Including the bacon.) Blend in the buttermilk and heavy cream and emulsify. Let cool completely, then refrigerate for several hours before serving, or overnight. When ready to serve, garnish with bacon bits, snipped chives, or other mashed potato-appropriate toppings, such as smoked salmon and/or crushed dill pickle potato chips. Or just drink it like a milkshake.